X-Git-Url: https://git.lttng.org/?p=lttng-tools.git;a=blobdiff_plain;f=src%2Fvendor%2Ffmt%2Fchrono.h;fp=src%2Fvendor%2Ffmt%2Fchrono.h;h=682efd8d21b1464dd40a641ba8096f8f2c915197;hp=0000000000000000000000000000000000000000;hb=05aa7e19ec97871aad18d7c9787c4c89611cd2cd;hpb=b3647fb885288c63d21478ea9a9c85685bc5c5f2

diff --git a/src/vendor/fmt/chrono.h b/src/vendor/fmt/chrono.h
new file mode 100644
index 000000000..682efd8d2
--- /dev/null
+++ b/src/vendor/fmt/chrono.h
@@ -0,0 +1,2067 @@
+// Formatting library for C++ - chrono support
+//
+// Copyright (c) 2012 - present, Victor Zverovich
+// All rights reserved.
+//
+// For the license information refer to format.h.
+
+#ifndef FMT_CHRONO_H_
+#define FMT_CHRONO_H_
+
+#include <algorithm>
+#include <chrono>
+#include <ctime>
+#include <iterator>
+#include <locale>
+#include <ostream>
+#include <type_traits>
+
+#include "format.h"
+
+FMT_BEGIN_NAMESPACE
+
+// Enable tzset.
+#ifndef FMT_USE_TZSET
+// UWP doesn't provide _tzset.
+#  if FMT_HAS_INCLUDE("winapifamily.h")
+#    include <winapifamily.h>
+#  endif
+#  if defined(_WIN32) && (!defined(WINAPI_FAMILY) || \
+                          (WINAPI_FAMILY == WINAPI_FAMILY_DESKTOP_APP))
+#    define FMT_USE_TZSET 1
+#  else
+#    define FMT_USE_TZSET 0
+#  endif
+#endif
+
+// Enable safe chrono durations, unless explicitly disabled.
+#ifndef FMT_SAFE_DURATION_CAST
+#  define FMT_SAFE_DURATION_CAST 1
+#endif
+#if FMT_SAFE_DURATION_CAST
+
+// For conversion between std::chrono::durations without undefined
+// behaviour or erroneous results.
+// This is a stripped down version of duration_cast, for inclusion in fmt.
+// See https://github.com/pauldreik/safe_duration_cast
+//
+// Copyright Paul Dreik 2019
+namespace safe_duration_cast {
+
+template <typename To, typename From,
+          FMT_ENABLE_IF(!std::is_same<From, To>::value &&
+                        std::numeric_limits<From>::is_signed ==
+                            std::numeric_limits<To>::is_signed)>
+FMT_CONSTEXPR To lossless_integral_conversion(const From from, int& ec) {
+  ec = 0;
+  using F = std::numeric_limits<From>;
+  using T = std::numeric_limits<To>;
+  static_assert(F::is_integer, "From must be integral");
+  static_assert(T::is_integer, "To must be integral");
+
+  // A and B are both signed, or both unsigned.
+  if (detail::const_check(F::digits <= T::digits)) {
+    // From fits in To without any problem.
+  } else {
+    // From does not always fit in To, resort to a dynamic check.
+    if (from < (T::min)() || from > (T::max)()) {
+      // outside range.
+      ec = 1;
+      return {};
+    }
+  }
+  return static_cast<To>(from);
+}
+
+/**
+ * converts From to To, without loss. If the dynamic value of from
+ * can't be converted to To without loss, ec is set.
+ */
+template <typename To, typename From,
+          FMT_ENABLE_IF(!std::is_same<From, To>::value &&
+                        std::numeric_limits<From>::is_signed !=
+                            std::numeric_limits<To>::is_signed)>
+FMT_CONSTEXPR To lossless_integral_conversion(const From from, int& ec) {
+  ec = 0;
+  using F = std::numeric_limits<From>;
+  using T = std::numeric_limits<To>;
+  static_assert(F::is_integer, "From must be integral");
+  static_assert(T::is_integer, "To must be integral");
+
+  if (detail::const_check(F::is_signed && !T::is_signed)) {
+    // From may be negative, not allowed!
+    if (fmt::detail::is_negative(from)) {
+      ec = 1;
+      return {};
+    }
+    // From is positive. Can it always fit in To?
+    if (detail::const_check(F::digits > T::digits) &&
+        from > static_cast<From>(detail::max_value<To>())) {
+      ec = 1;
+      return {};
+    }
+  }
+
+  if (detail::const_check(!F::is_signed && T::is_signed &&
+                          F::digits >= T::digits) &&
+      from > static_cast<From>(detail::max_value<To>())) {
+    ec = 1;
+    return {};
+  }
+  return static_cast<To>(from);  // Lossless conversion.
+}
+
+template <typename To, typename From,
+          FMT_ENABLE_IF(std::is_same<From, To>::value)>
+FMT_CONSTEXPR To lossless_integral_conversion(const From from, int& ec) {
+  ec = 0;
+  return from;
+}  // function
+
+// clang-format off
+/**
+ * converts From to To if possible, otherwise ec is set.
+ *
+ * input                            |    output
+ * ---------------------------------|---------------
+ * NaN                              | NaN
+ * Inf                              | Inf
+ * normal, fits in output           | converted (possibly lossy)
+ * normal, does not fit in output   | ec is set
+ * subnormal                        | best effort
+ * -Inf                             | -Inf
+ */
+// clang-format on
+template <typename To, typename From,
+          FMT_ENABLE_IF(!std::is_same<From, To>::value)>
+FMT_CONSTEXPR To safe_float_conversion(const From from, int& ec) {
+  ec = 0;
+  using T = std::numeric_limits<To>;
+  static_assert(std::is_floating_point<From>::value, "From must be floating");
+  static_assert(std::is_floating_point<To>::value, "To must be floating");
+
+  // catch the only happy case
+  if (std::isfinite(from)) {
+    if (from >= T::lowest() && from <= (T::max)()) {
+      return static_cast<To>(from);
+    }
+    // not within range.
+    ec = 1;
+    return {};
+  }
+
+  // nan and inf will be preserved
+  return static_cast<To>(from);
+}  // function
+
+template <typename To, typename From,
+          FMT_ENABLE_IF(std::is_same<From, To>::value)>
+FMT_CONSTEXPR To safe_float_conversion(const From from, int& ec) {
+  ec = 0;
+  static_assert(std::is_floating_point<From>::value, "From must be floating");
+  return from;
+}
+
+/**
+ * safe duration cast between integral durations
+ */
+template <typename To, typename FromRep, typename FromPeriod,
+          FMT_ENABLE_IF(std::is_integral<FromRep>::value),
+          FMT_ENABLE_IF(std::is_integral<typename To::rep>::value)>
+To safe_duration_cast(std::chrono::duration<FromRep, FromPeriod> from,
+                      int& ec) {
+  using From = std::chrono::duration<FromRep, FromPeriod>;
+  ec = 0;
+  // the basic idea is that we need to convert from count() in the from type
+  // to count() in the To type, by multiplying it with this:
+  struct Factor
+      : std::ratio_divide<typename From::period, typename To::period> {};
+
+  static_assert(Factor::num > 0, "num must be positive");
+  static_assert(Factor::den > 0, "den must be positive");
+
+  // the conversion is like this: multiply from.count() with Factor::num
+  // /Factor::den and convert it to To::rep, all this without
+  // overflow/underflow. let's start by finding a suitable type that can hold
+  // both To, From and Factor::num
+  using IntermediateRep =
+      typename std::common_type<typename From::rep, typename To::rep,
+                                decltype(Factor::num)>::type;
+
+  // safe conversion to IntermediateRep
+  IntermediateRep count =
+      lossless_integral_conversion<IntermediateRep>(from.count(), ec);
+  if (ec) return {};
+  // multiply with Factor::num without overflow or underflow
+  if (detail::const_check(Factor::num != 1)) {
+    const auto max1 = detail::max_value<IntermediateRep>() / Factor::num;
+    if (count > max1) {
+      ec = 1;
+      return {};
+    }
+    const auto min1 =
+        (std::numeric_limits<IntermediateRep>::min)() / Factor::num;
+    if (count < min1) {
+      ec = 1;
+      return {};
+    }
+    count *= Factor::num;
+  }
+
+  if (detail::const_check(Factor::den != 1)) count /= Factor::den;
+  auto tocount = lossless_integral_conversion<typename To::rep>(count, ec);
+  return ec ? To() : To(tocount);
+}
+
+/**
+ * safe duration_cast between floating point durations
+ */
+template <typename To, typename FromRep, typename FromPeriod,
+          FMT_ENABLE_IF(std::is_floating_point<FromRep>::value),
+          FMT_ENABLE_IF(std::is_floating_point<typename To::rep>::value)>
+To safe_duration_cast(std::chrono::duration<FromRep, FromPeriod> from,
+                      int& ec) {
+  using From = std::chrono::duration<FromRep, FromPeriod>;
+  ec = 0;
+  if (std::isnan(from.count())) {
+    // nan in, gives nan out. easy.
+    return To{std::numeric_limits<typename To::rep>::quiet_NaN()};
+  }
+  // maybe we should also check if from is denormal, and decide what to do about
+  // it.
+
+  // +-inf should be preserved.
+  if (std::isinf(from.count())) {
+    return To{from.count()};
+  }
+
+  // the basic idea is that we need to convert from count() in the from type
+  // to count() in the To type, by multiplying it with this:
+  struct Factor
+      : std::ratio_divide<typename From::period, typename To::period> {};
+
+  static_assert(Factor::num > 0, "num must be positive");
+  static_assert(Factor::den > 0, "den must be positive");
+
+  // the conversion is like this: multiply from.count() with Factor::num
+  // /Factor::den and convert it to To::rep, all this without
+  // overflow/underflow. let's start by finding a suitable type that can hold
+  // both To, From and Factor::num
+  using IntermediateRep =
+      typename std::common_type<typename From::rep, typename To::rep,
+                                decltype(Factor::num)>::type;
+
+  // force conversion of From::rep -> IntermediateRep to be safe,
+  // even if it will never happen be narrowing in this context.
+  IntermediateRep count =
+      safe_float_conversion<IntermediateRep>(from.count(), ec);
+  if (ec) {
+    return {};
+  }
+
+  // multiply with Factor::num without overflow or underflow
+  if (detail::const_check(Factor::num != 1)) {
+    constexpr auto max1 = detail::max_value<IntermediateRep>() /
+                          static_cast<IntermediateRep>(Factor::num);
+    if (count > max1) {
+      ec = 1;
+      return {};
+    }
+    constexpr auto min1 = std::numeric_limits<IntermediateRep>::lowest() /
+                          static_cast<IntermediateRep>(Factor::num);
+    if (count < min1) {
+      ec = 1;
+      return {};
+    }
+    count *= static_cast<IntermediateRep>(Factor::num);
+  }
+
+  // this can't go wrong, right? den>0 is checked earlier.
+  if (detail::const_check(Factor::den != 1)) {
+    using common_t = typename std::common_type<IntermediateRep, intmax_t>::type;
+    count /= static_cast<common_t>(Factor::den);
+  }
+
+  // convert to the to type, safely
+  using ToRep = typename To::rep;
+
+  const ToRep tocount = safe_float_conversion<ToRep>(count, ec);
+  if (ec) {
+    return {};
+  }
+  return To{tocount};
+}
+}  // namespace safe_duration_cast
+#endif
+
+// Prevents expansion of a preceding token as a function-style macro.
+// Usage: f FMT_NOMACRO()
+#define FMT_NOMACRO
+
+namespace detail {
+template <typename T = void> struct null {};
+inline null<> localtime_r FMT_NOMACRO(...) { return null<>(); }
+inline null<> localtime_s(...) { return null<>(); }
+inline null<> gmtime_r(...) { return null<>(); }
+inline null<> gmtime_s(...) { return null<>(); }
+
+inline const std::locale& get_classic_locale() {
+  static const auto& locale = std::locale::classic();
+  return locale;
+}
+
+template <typename CodeUnit> struct codecvt_result {
+  static constexpr const size_t max_size = 32;
+  CodeUnit buf[max_size];
+  CodeUnit* end;
+};
+template <typename CodeUnit>
+constexpr const size_t codecvt_result<CodeUnit>::max_size;
+
+template <typename CodeUnit>
+void write_codecvt(codecvt_result<CodeUnit>& out, string_view in_buf,
+                   const std::locale& loc) {
+  using codecvt = std::codecvt<CodeUnit, char, std::mbstate_t>;
+#if FMT_CLANG_VERSION
+#  pragma clang diagnostic push
+#  pragma clang diagnostic ignored "-Wdeprecated"
+  auto& f = std::use_facet<codecvt>(loc);
+#  pragma clang diagnostic pop
+#else
+  auto& f = std::use_facet<codecvt>(loc);
+#endif
+  auto mb = std::mbstate_t();
+  const char* from_next = nullptr;
+  auto result = f.in(mb, in_buf.begin(), in_buf.end(), from_next,
+                     std::begin(out.buf), std::end(out.buf), out.end);
+  if (result != std::codecvt_base::ok)
+    FMT_THROW(format_error("failed to format time"));
+}
+
+template <typename OutputIt>
+auto write_encoded_tm_str(OutputIt out, string_view in, const std::locale& loc)
+    -> OutputIt {
+  if (detail::is_utf8() && loc != get_classic_locale()) {
+    // char16_t and char32_t codecvts are broken in MSVC (linkage errors) and
+    // gcc-4.
+#if FMT_MSC_VER != 0 || \
+    (defined(__GLIBCXX__) && !defined(_GLIBCXX_USE_DUAL_ABI))
+    // The _GLIBCXX_USE_DUAL_ABI macro is always defined in libstdc++ from gcc-5
+    // and newer.
+    using code_unit = wchar_t;
+#else
+    using code_unit = char32_t;
+#endif
+
+    using unit_t = codecvt_result<code_unit>;
+    unit_t unit;
+    write_codecvt(unit, in, loc);
+    // In UTF-8 is used one to four one-byte code units.
+    auto&& buf = basic_memory_buffer<char, unit_t::max_size * 4>();
+    for (code_unit* p = unit.buf; p != unit.end; ++p) {
+      uint32_t c = static_cast<uint32_t>(*p);
+      if (sizeof(code_unit) == 2 && c >= 0xd800 && c <= 0xdfff) {
+        // surrogate pair
+        ++p;
+        if (p == unit.end || (c & 0xfc00) != 0xd800 ||
+            (*p & 0xfc00) != 0xdc00) {
+          FMT_THROW(format_error("failed to format time"));
+        }
+        c = (c << 10) + static_cast<uint32_t>(*p) - 0x35fdc00;
+      }
+      if (c < 0x80) {
+        buf.push_back(static_cast<char>(c));
+      } else if (c < 0x800) {
+        buf.push_back(static_cast<char>(0xc0 | (c >> 6)));
+        buf.push_back(static_cast<char>(0x80 | (c & 0x3f)));
+      } else if ((c >= 0x800 && c <= 0xd7ff) || (c >= 0xe000 && c <= 0xffff)) {
+        buf.push_back(static_cast<char>(0xe0 | (c >> 12)));
+        buf.push_back(static_cast<char>(0x80 | ((c & 0xfff) >> 6)));
+        buf.push_back(static_cast<char>(0x80 | (c & 0x3f)));
+      } else if (c >= 0x10000 && c <= 0x10ffff) {
+        buf.push_back(static_cast<char>(0xf0 | (c >> 18)));
+        buf.push_back(static_cast<char>(0x80 | ((c & 0x3ffff) >> 12)));
+        buf.push_back(static_cast<char>(0x80 | ((c & 0xfff) >> 6)));
+        buf.push_back(static_cast<char>(0x80 | (c & 0x3f)));
+      } else {
+        FMT_THROW(format_error("failed to format time"));
+      }
+    }
+    return copy_str<char>(buf.data(), buf.data() + buf.size(), out);
+  }
+  return copy_str<char>(in.data(), in.data() + in.size(), out);
+}
+
+template <typename Char, typename OutputIt,
+          FMT_ENABLE_IF(!std::is_same<Char, char>::value)>
+auto write_tm_str(OutputIt out, string_view sv, const std::locale& loc)
+    -> OutputIt {
+  codecvt_result<Char> unit;
+  write_codecvt(unit, sv, loc);
+  return copy_str<Char>(unit.buf, unit.end, out);
+}
+
+template <typename Char, typename OutputIt,
+          FMT_ENABLE_IF(std::is_same<Char, char>::value)>
+auto write_tm_str(OutputIt out, string_view sv, const std::locale& loc)
+    -> OutputIt {
+  return write_encoded_tm_str(out, sv, loc);
+}
+
+template <typename Char>
+inline void do_write(buffer<Char>& buf, const std::tm& time,
+                     const std::locale& loc, char format, char modifier) {
+  auto&& format_buf = formatbuf<std::basic_streambuf<Char>>(buf);
+  auto&& os = std::basic_ostream<Char>(&format_buf);
+  os.imbue(loc);
+  using iterator = std::ostreambuf_iterator<Char>;
+  const auto& facet = std::use_facet<std::time_put<Char, iterator>>(loc);
+  auto end = facet.put(os, os, Char(' '), &time, format, modifier);
+  if (end.failed()) FMT_THROW(format_error("failed to format time"));
+}
+
+template <typename Char, typename OutputIt,
+          FMT_ENABLE_IF(!std::is_same<Char, char>::value)>
+auto write(OutputIt out, const std::tm& time, const std::locale& loc,
+           char format, char modifier = 0) -> OutputIt {
+  auto&& buf = get_buffer<Char>(out);
+  do_write<Char>(buf, time, loc, format, modifier);
+  return buf.out();
+}
+
+template <typename Char, typename OutputIt,
+          FMT_ENABLE_IF(std::is_same<Char, char>::value)>
+auto write(OutputIt out, const std::tm& time, const std::locale& loc,
+           char format, char modifier = 0) -> OutputIt {
+  auto&& buf = basic_memory_buffer<Char>();
+  do_write<char>(buf, time, loc, format, modifier);
+  return write_encoded_tm_str(out, string_view(buf.data(), buf.size()), loc);
+}
+
+}  // namespace detail
+
+FMT_MODULE_EXPORT_BEGIN
+
+/**
+  Converts given time since epoch as ``std::time_t`` value into calendar time,
+  expressed in local time. Unlike ``std::localtime``, this function is
+  thread-safe on most platforms.
+ */
+inline std::tm localtime(std::time_t time) {
+  struct dispatcher {
+    std::time_t time_;
+    std::tm tm_;
+
+    dispatcher(std::time_t t) : time_(t) {}
+
+    bool run() {
+      using namespace fmt::detail;
+      return handle(localtime_r(&time_, &tm_));
+    }
+
+    bool handle(std::tm* tm) { return tm != nullptr; }
+
+    bool handle(detail::null<>) {
+      using namespace fmt::detail;
+      return fallback(localtime_s(&tm_, &time_));
+    }
+
+    bool fallback(int res) { return res == 0; }
+
+#if !FMT_MSC_VER
+    bool fallback(detail::null<>) {
+      using namespace fmt::detail;
+      std::tm* tm = std::localtime(&time_);
+      if (tm) tm_ = *tm;
+      return tm != nullptr;
+    }
+#endif
+  };
+  dispatcher lt(time);
+  // Too big time values may be unsupported.
+  if (!lt.run()) FMT_THROW(format_error("time_t value out of range"));
+  return lt.tm_;
+}
+
+inline std::tm localtime(
+    std::chrono::time_point<std::chrono::system_clock> time_point) {
+  return localtime(std::chrono::system_clock::to_time_t(time_point));
+}
+
+/**
+  Converts given time since epoch as ``std::time_t`` value into calendar time,
+  expressed in Coordinated Universal Time (UTC). Unlike ``std::gmtime``, this
+  function is thread-safe on most platforms.
+ */
+inline std::tm gmtime(std::time_t time) {
+  struct dispatcher {
+    std::time_t time_;
+    std::tm tm_;
+
+    dispatcher(std::time_t t) : time_(t) {}
+
+    bool run() {
+      using namespace fmt::detail;
+      return handle(gmtime_r(&time_, &tm_));
+    }
+
+    bool handle(std::tm* tm) { return tm != nullptr; }
+
+    bool handle(detail::null<>) {
+      using namespace fmt::detail;
+      return fallback(gmtime_s(&tm_, &time_));
+    }
+
+    bool fallback(int res) { return res == 0; }
+
+#if !FMT_MSC_VER
+    bool fallback(detail::null<>) {
+      std::tm* tm = std::gmtime(&time_);
+      if (tm) tm_ = *tm;
+      return tm != nullptr;
+    }
+#endif
+  };
+  dispatcher gt(time);
+  // Too big time values may be unsupported.
+  if (!gt.run()) FMT_THROW(format_error("time_t value out of range"));
+  return gt.tm_;
+}
+
+inline std::tm gmtime(
+    std::chrono::time_point<std::chrono::system_clock> time_point) {
+  return gmtime(std::chrono::system_clock::to_time_t(time_point));
+}
+
+FMT_BEGIN_DETAIL_NAMESPACE
+
+// Writes two-digit numbers a, b and c separated by sep to buf.
+// The method by Pavel Novikov based on
+// https://johnnylee-sde.github.io/Fast-unsigned-integer-to-time-string/.
+inline void write_digit2_separated(char* buf, unsigned a, unsigned b,
+                                   unsigned c, char sep) {
+  unsigned long long digits =
+      a | (b << 24) | (static_cast<unsigned long long>(c) << 48);
+  // Convert each value to BCD.
+  // We have x = a * 10 + b and we want to convert it to BCD y = a * 16 + b.
+  // The difference is
+  //   y - x = a * 6
+  // a can be found from x:
+  //   a = floor(x / 10)
+  // then
+  //   y = x + a * 6 = x + floor(x / 10) * 6
+  // floor(x / 10) is (x * 205) >> 11 (needs 16 bits).
+  digits += (((digits * 205) >> 11) & 0x000f00000f00000f) * 6;
+  // Put low nibbles to high bytes and high nibbles to low bytes.
+  digits = ((digits & 0x00f00000f00000f0) >> 4) |
+           ((digits & 0x000f00000f00000f) << 8);
+  auto usep = static_cast<unsigned long long>(sep);
+  // Add ASCII '0' to each digit byte and insert separators.
+  digits |= 0x3030003030003030 | (usep << 16) | (usep << 40);
+
+  constexpr const size_t len = 8;
+  if (const_check(is_big_endian())) {
+    char tmp[len];
+    memcpy(tmp, &digits, len);
+    std::reverse_copy(tmp, tmp + len, buf);
+  } else {
+    memcpy(buf, &digits, len);
+  }
+}
+
+template <typename Period> FMT_CONSTEXPR inline const char* get_units() {
+  if (std::is_same<Period, std::atto>::value) return "as";
+  if (std::is_same<Period, std::femto>::value) return "fs";
+  if (std::is_same<Period, std::pico>::value) return "ps";
+  if (std::is_same<Period, std::nano>::value) return "ns";
+  if (std::is_same<Period, std::micro>::value) return "µs";
+  if (std::is_same<Period, std::milli>::value) return "ms";
+  if (std::is_same<Period, std::centi>::value) return "cs";
+  if (std::is_same<Period, std::deci>::value) return "ds";
+  if (std::is_same<Period, std::ratio<1>>::value) return "s";
+  if (std::is_same<Period, std::deca>::value) return "das";
+  if (std::is_same<Period, std::hecto>::value) return "hs";
+  if (std::is_same<Period, std::kilo>::value) return "ks";
+  if (std::is_same<Period, std::mega>::value) return "Ms";
+  if (std::is_same<Period, std::giga>::value) return "Gs";
+  if (std::is_same<Period, std::tera>::value) return "Ts";
+  if (std::is_same<Period, std::peta>::value) return "Ps";
+  if (std::is_same<Period, std::exa>::value) return "Es";
+  if (std::is_same<Period, std::ratio<60>>::value) return "m";
+  if (std::is_same<Period, std::ratio<3600>>::value) return "h";
+  return nullptr;
+}
+
+enum class numeric_system {
+  standard,
+  // Alternative numeric system, e.g. 十二 instead of 12 in ja_JP locale.
+  alternative
+};
+
+// Parses a put_time-like format string and invokes handler actions.
+template <typename Char, typename Handler>
+FMT_CONSTEXPR const Char* parse_chrono_format(const Char* begin,
+                                              const Char* end,
+                                              Handler&& handler) {
+  auto ptr = begin;
+  while (ptr != end) {
+    auto c = *ptr;
+    if (c == '}') break;
+    if (c != '%') {
+      ++ptr;
+      continue;
+    }
+    if (begin != ptr) handler.on_text(begin, ptr);
+    ++ptr;  // consume '%'
+    if (ptr == end) FMT_THROW(format_error("invalid format"));
+    c = *ptr++;
+    switch (c) {
+    case '%':
+      handler.on_text(ptr - 1, ptr);
+      break;
+    case 'n': {
+      const Char newline[] = {'\n'};
+      handler.on_text(newline, newline + 1);
+      break;
+    }
+    case 't': {
+      const Char tab[] = {'\t'};
+      handler.on_text(tab, tab + 1);
+      break;
+    }
+    // Year:
+    case 'Y':
+      handler.on_year(numeric_system::standard);
+      break;
+    case 'y':
+      handler.on_short_year(numeric_system::standard);
+      break;
+    case 'C':
+      handler.on_century(numeric_system::standard);
+      break;
+    case 'G':
+      handler.on_iso_week_based_year();
+      break;
+    case 'g':
+      handler.on_iso_week_based_short_year();
+      break;
+    // Day of the week:
+    case 'a':
+      handler.on_abbr_weekday();
+      break;
+    case 'A':
+      handler.on_full_weekday();
+      break;
+    case 'w':
+      handler.on_dec0_weekday(numeric_system::standard);
+      break;
+    case 'u':
+      handler.on_dec1_weekday(numeric_system::standard);
+      break;
+    // Month:
+    case 'b':
+    case 'h':
+      handler.on_abbr_month();
+      break;
+    case 'B':
+      handler.on_full_month();
+      break;
+    case 'm':
+      handler.on_dec_month(numeric_system::standard);
+      break;
+    // Day of the year/month:
+    case 'U':
+      handler.on_dec0_week_of_year(numeric_system::standard);
+      break;
+    case 'W':
+      handler.on_dec1_week_of_year(numeric_system::standard);
+      break;
+    case 'V':
+      handler.on_iso_week_of_year(numeric_system::standard);
+      break;
+    case 'j':
+      handler.on_day_of_year();
+      break;
+    case 'd':
+      handler.on_day_of_month(numeric_system::standard);
+      break;
+    case 'e':
+      handler.on_day_of_month_space(numeric_system::standard);
+      break;
+    // Hour, minute, second:
+    case 'H':
+      handler.on_24_hour(numeric_system::standard);
+      break;
+    case 'I':
+      handler.on_12_hour(numeric_system::standard);
+      break;
+    case 'M':
+      handler.on_minute(numeric_system::standard);
+      break;
+    case 'S':
+      handler.on_second(numeric_system::standard);
+      break;
+    // Other:
+    case 'c':
+      handler.on_datetime(numeric_system::standard);
+      break;
+    case 'x':
+      handler.on_loc_date(numeric_system::standard);
+      break;
+    case 'X':
+      handler.on_loc_time(numeric_system::standard);
+      break;
+    case 'D':
+      handler.on_us_date();
+      break;
+    case 'F':
+      handler.on_iso_date();
+      break;
+    case 'r':
+      handler.on_12_hour_time();
+      break;
+    case 'R':
+      handler.on_24_hour_time();
+      break;
+    case 'T':
+      handler.on_iso_time();
+      break;
+    case 'p':
+      handler.on_am_pm();
+      break;
+    case 'Q':
+      handler.on_duration_value();
+      break;
+    case 'q':
+      handler.on_duration_unit();
+      break;
+    case 'z':
+      handler.on_utc_offset();
+      break;
+    case 'Z':
+      handler.on_tz_name();
+      break;
+    // Alternative representation:
+    case 'E': {
+      if (ptr == end) FMT_THROW(format_error("invalid format"));
+      c = *ptr++;
+      switch (c) {
+      case 'Y':
+        handler.on_year(numeric_system::alternative);
+        break;
+      case 'y':
+        handler.on_offset_year();
+        break;
+      case 'C':
+        handler.on_century(numeric_system::alternative);
+        break;
+      case 'c':
+        handler.on_datetime(numeric_system::alternative);
+        break;
+      case 'x':
+        handler.on_loc_date(numeric_system::alternative);
+        break;
+      case 'X':
+        handler.on_loc_time(numeric_system::alternative);
+        break;
+      default:
+        FMT_THROW(format_error("invalid format"));
+      }
+      break;
+    }
+    case 'O':
+      if (ptr == end) FMT_THROW(format_error("invalid format"));
+      c = *ptr++;
+      switch (c) {
+      case 'y':
+        handler.on_short_year(numeric_system::alternative);
+        break;
+      case 'm':
+        handler.on_dec_month(numeric_system::alternative);
+        break;
+      case 'U':
+        handler.on_dec0_week_of_year(numeric_system::alternative);
+        break;
+      case 'W':
+        handler.on_dec1_week_of_year(numeric_system::alternative);
+        break;
+      case 'V':
+        handler.on_iso_week_of_year(numeric_system::alternative);
+        break;
+      case 'd':
+        handler.on_day_of_month(numeric_system::alternative);
+        break;
+      case 'e':
+        handler.on_day_of_month_space(numeric_system::alternative);
+        break;
+      case 'w':
+        handler.on_dec0_weekday(numeric_system::alternative);
+        break;
+      case 'u':
+        handler.on_dec1_weekday(numeric_system::alternative);
+        break;
+      case 'H':
+        handler.on_24_hour(numeric_system::alternative);
+        break;
+      case 'I':
+        handler.on_12_hour(numeric_system::alternative);
+        break;
+      case 'M':
+        handler.on_minute(numeric_system::alternative);
+        break;
+      case 'S':
+        handler.on_second(numeric_system::alternative);
+        break;
+      default:
+        FMT_THROW(format_error("invalid format"));
+      }
+      break;
+    default:
+      FMT_THROW(format_error("invalid format"));
+    }
+    begin = ptr;
+  }
+  if (begin != ptr) handler.on_text(begin, ptr);
+  return ptr;
+}
+
+template <typename Derived> struct null_chrono_spec_handler {
+  FMT_CONSTEXPR void unsupported() {
+    static_cast<Derived*>(this)->unsupported();
+  }
+  FMT_CONSTEXPR void on_year(numeric_system) { unsupported(); }
+  FMT_CONSTEXPR void on_short_year(numeric_system) { unsupported(); }
+  FMT_CONSTEXPR void on_offset_year() { unsupported(); }
+  FMT_CONSTEXPR void on_century(numeric_system) { unsupported(); }
+  FMT_CONSTEXPR void on_iso_week_based_year() { unsupported(); }
+  FMT_CONSTEXPR void on_iso_week_based_short_year() { unsupported(); }
+  FMT_CONSTEXPR void on_abbr_weekday() { unsupported(); }
+  FMT_CONSTEXPR void on_full_weekday() { unsupported(); }
+  FMT_CONSTEXPR void on_dec0_weekday(numeric_system) { unsupported(); }
+  FMT_CONSTEXPR void on_dec1_weekday(numeric_system) { unsupported(); }
+  FMT_CONSTEXPR void on_abbr_month() { unsupported(); }
+  FMT_CONSTEXPR void on_full_month() { unsupported(); }
+  FMT_CONSTEXPR void on_dec_month(numeric_system) { unsupported(); }
+  FMT_CONSTEXPR void on_dec0_week_of_year(numeric_system) { unsupported(); }
+  FMT_CONSTEXPR void on_dec1_week_of_year(numeric_system) { unsupported(); }
+  FMT_CONSTEXPR void on_iso_week_of_year(numeric_system) { unsupported(); }
+  FMT_CONSTEXPR void on_day_of_year() { unsupported(); }
+  FMT_CONSTEXPR void on_day_of_month(numeric_system) { unsupported(); }
+  FMT_CONSTEXPR void on_day_of_month_space(numeric_system) { unsupported(); }
+  FMT_CONSTEXPR void on_24_hour(numeric_system) { unsupported(); }
+  FMT_CONSTEXPR void on_12_hour(numeric_system) { unsupported(); }
+  FMT_CONSTEXPR void on_minute(numeric_system) { unsupported(); }
+  FMT_CONSTEXPR void on_second(numeric_system) { unsupported(); }
+  FMT_CONSTEXPR void on_datetime(numeric_system) { unsupported(); }
+  FMT_CONSTEXPR void on_loc_date(numeric_system) { unsupported(); }
+  FMT_CONSTEXPR void on_loc_time(numeric_system) { unsupported(); }
+  FMT_CONSTEXPR void on_us_date() { unsupported(); }
+  FMT_CONSTEXPR void on_iso_date() { unsupported(); }
+  FMT_CONSTEXPR void on_12_hour_time() { unsupported(); }
+  FMT_CONSTEXPR void on_24_hour_time() { unsupported(); }
+  FMT_CONSTEXPR void on_iso_time() { unsupported(); }
+  FMT_CONSTEXPR void on_am_pm() { unsupported(); }
+  FMT_CONSTEXPR void on_duration_value() { unsupported(); }
+  FMT_CONSTEXPR void on_duration_unit() { unsupported(); }
+  FMT_CONSTEXPR void on_utc_offset() { unsupported(); }
+  FMT_CONSTEXPR void on_tz_name() { unsupported(); }
+};
+
+struct tm_format_checker : null_chrono_spec_handler<tm_format_checker> {
+  FMT_NORETURN void unsupported() { FMT_THROW(format_error("no format")); }
+
+  template <typename Char>
+  FMT_CONSTEXPR void on_text(const Char*, const Char*) {}
+  FMT_CONSTEXPR void on_year(numeric_system) {}
+  FMT_CONSTEXPR void on_short_year(numeric_system) {}
+  FMT_CONSTEXPR void on_offset_year() {}
+  FMT_CONSTEXPR void on_century(numeric_system) {}
+  FMT_CONSTEXPR void on_iso_week_based_year() {}
+  FMT_CONSTEXPR void on_iso_week_based_short_year() {}
+  FMT_CONSTEXPR void on_abbr_weekday() {}
+  FMT_CONSTEXPR void on_full_weekday() {}
+  FMT_CONSTEXPR void on_dec0_weekday(numeric_system) {}
+  FMT_CONSTEXPR void on_dec1_weekday(numeric_system) {}
+  FMT_CONSTEXPR void on_abbr_month() {}
+  FMT_CONSTEXPR void on_full_month() {}
+  FMT_CONSTEXPR void on_dec_month(numeric_system) {}
+  FMT_CONSTEXPR void on_dec0_week_of_year(numeric_system) {}
+  FMT_CONSTEXPR void on_dec1_week_of_year(numeric_system) {}
+  FMT_CONSTEXPR void on_iso_week_of_year(numeric_system) {}
+  FMT_CONSTEXPR void on_day_of_year() {}
+  FMT_CONSTEXPR void on_day_of_month(numeric_system) {}
+  FMT_CONSTEXPR void on_day_of_month_space(numeric_system) {}
+  FMT_CONSTEXPR void on_24_hour(numeric_system) {}
+  FMT_CONSTEXPR void on_12_hour(numeric_system) {}
+  FMT_CONSTEXPR void on_minute(numeric_system) {}
+  FMT_CONSTEXPR void on_second(numeric_system) {}
+  FMT_CONSTEXPR void on_datetime(numeric_system) {}
+  FMT_CONSTEXPR void on_loc_date(numeric_system) {}
+  FMT_CONSTEXPR void on_loc_time(numeric_system) {}
+  FMT_CONSTEXPR void on_us_date() {}
+  FMT_CONSTEXPR void on_iso_date() {}
+  FMT_CONSTEXPR void on_12_hour_time() {}
+  FMT_CONSTEXPR void on_24_hour_time() {}
+  FMT_CONSTEXPR void on_iso_time() {}
+  FMT_CONSTEXPR void on_am_pm() {}
+  FMT_CONSTEXPR void on_utc_offset() {}
+  FMT_CONSTEXPR void on_tz_name() {}
+};
+
+inline const char* tm_wday_full_name(int wday) {
+  static constexpr const char* full_name_list[] = {
+      "Sunday",   "Monday", "Tuesday", "Wednesday",
+      "Thursday", "Friday", "Saturday"};
+  return wday >= 0 && wday <= 6 ? full_name_list[wday] : "?";
+}
+inline const char* tm_wday_short_name(int wday) {
+  static constexpr const char* short_name_list[] = {"Sun", "Mon", "Tue", "Wed",
+                                                    "Thu", "Fri", "Sat"};
+  return wday >= 0 && wday <= 6 ? short_name_list[wday] : "???";
+}
+
+inline const char* tm_mon_full_name(int mon) {
+  static constexpr const char* full_name_list[] = {
+      "January", "February", "March",     "April",   "May",      "June",
+      "July",    "August",   "September", "October", "November", "December"};
+  return mon >= 0 && mon <= 11 ? full_name_list[mon] : "?";
+}
+inline const char* tm_mon_short_name(int mon) {
+  static constexpr const char* short_name_list[] = {
+      "Jan", "Feb", "Mar", "Apr", "May", "Jun",
+      "Jul", "Aug", "Sep", "Oct", "Nov", "Dec",
+  };
+  return mon >= 0 && mon <= 11 ? short_name_list[mon] : "???";
+}
+
+template <typename T, typename = void>
+struct has_member_data_tm_gmtoff : std::false_type {};
+template <typename T>
+struct has_member_data_tm_gmtoff<T, void_t<decltype(T::tm_gmtoff)>>
+    : std::true_type {};
+
+template <typename T, typename = void>
+struct has_member_data_tm_zone : std::false_type {};
+template <typename T>
+struct has_member_data_tm_zone<T, void_t<decltype(T::tm_zone)>>
+    : std::true_type {};
+
+#if FMT_USE_TZSET
+inline void tzset_once() {
+  static bool init = []() -> bool {
+    _tzset();
+    return true;
+  }();
+  ignore_unused(init);
+}
+#endif
+
+template <typename OutputIt, typename Char> class tm_writer {
+ private:
+  static constexpr int days_per_week = 7;
+
+  const std::locale& loc_;
+  const bool is_classic_;
+  OutputIt out_;
+  const std::tm& tm_;
+
+  auto tm_sec() const noexcept -> int {
+    FMT_ASSERT(tm_.tm_sec >= 0 && tm_.tm_sec <= 61, "");
+    return tm_.tm_sec;
+  }
+  auto tm_min() const noexcept -> int {
+    FMT_ASSERT(tm_.tm_min >= 0 && tm_.tm_min <= 59, "");
+    return tm_.tm_min;
+  }
+  auto tm_hour() const noexcept -> int {
+    FMT_ASSERT(tm_.tm_hour >= 0 && tm_.tm_hour <= 23, "");
+    return tm_.tm_hour;
+  }
+  auto tm_mday() const noexcept -> int {
+    FMT_ASSERT(tm_.tm_mday >= 1 && tm_.tm_mday <= 31, "");
+    return tm_.tm_mday;
+  }
+  auto tm_mon() const noexcept -> int {
+    FMT_ASSERT(tm_.tm_mon >= 0 && tm_.tm_mon <= 11, "");
+    return tm_.tm_mon;
+  }
+  auto tm_year() const noexcept -> long long { return 1900ll + tm_.tm_year; }
+  auto tm_wday() const noexcept -> int {
+    FMT_ASSERT(tm_.tm_wday >= 0 && tm_.tm_wday <= 6, "");
+    return tm_.tm_wday;
+  }
+  auto tm_yday() const noexcept -> int {
+    FMT_ASSERT(tm_.tm_yday >= 0 && tm_.tm_yday <= 365, "");
+    return tm_.tm_yday;
+  }
+
+  auto tm_hour12() const noexcept -> int {
+    const auto h = tm_hour();
+    const auto z = h < 12 ? h : h - 12;
+    return z == 0 ? 12 : z;
+  }
+
+  // POSIX and the C Standard are unclear or inconsistent about what %C and %y
+  // do if the year is negative or exceeds 9999. Use the convention that %C
+  // concatenated with %y yields the same output as %Y, and that %Y contains at
+  // least 4 characters, with more only if necessary.
+  auto split_year_lower(long long year) const noexcept -> int {
+    auto l = year % 100;
+    if (l < 0) l = -l;  // l in [0, 99]
+    return static_cast<int>(l);
+  }
+
+  // Algorithm:
+  // https://en.wikipedia.org/wiki/ISO_week_date#Calculating_the_week_number_from_a_month_and_day_of_the_month_or_ordinal_date
+  auto iso_year_weeks(long long curr_year) const noexcept -> int {
+    const auto prev_year = curr_year - 1;
+    const auto curr_p =
+        (curr_year + curr_year / 4 - curr_year / 100 + curr_year / 400) %
+        days_per_week;
+    const auto prev_p =
+        (prev_year + prev_year / 4 - prev_year / 100 + prev_year / 400) %
+        days_per_week;
+    return 52 + ((curr_p == 4 || prev_p == 3) ? 1 : 0);
+  }
+  auto iso_week_num(int tm_yday, int tm_wday) const noexcept -> int {
+    return (tm_yday + 11 - (tm_wday == 0 ? days_per_week : tm_wday)) /
+           days_per_week;
+  }
+  auto tm_iso_week_year() const noexcept -> long long {
+    const auto year = tm_year();
+    const auto w = iso_week_num(tm_yday(), tm_wday());
+    if (w < 1) return year - 1;
+    if (w > iso_year_weeks(year)) return year + 1;
+    return year;
+  }
+  auto tm_iso_week_of_year() const noexcept -> int {
+    const auto year = tm_year();
+    const auto w = iso_week_num(tm_yday(), tm_wday());
+    if (w < 1) return iso_year_weeks(year - 1);
+    if (w > iso_year_weeks(year)) return 1;
+    return w;
+  }
+
+  void write1(int value) {
+    *out_++ = static_cast<char>('0' + to_unsigned(value) % 10);
+  }
+  void write2(int value) {
+    const char* d = digits2(to_unsigned(value) % 100);
+    *out_++ = *d++;
+    *out_++ = *d;
+  }
+
+  void write_year_extended(long long year) {
+    // At least 4 characters.
+    int width = 4;
+    if (year < 0) {
+      *out_++ = '-';
+      year = 0 - year;
+      --width;
+    }
+    uint32_or_64_or_128_t<long long> n = to_unsigned(year);
+    const int num_digits = count_digits(n);
+    if (width > num_digits) out_ = std::fill_n(out_, width - num_digits, '0');
+    out_ = format_decimal<Char>(out_, n, num_digits).end;
+  }
+  void write_year(long long year) {
+    if (year >= 0 && year < 10000) {
+      write2(static_cast<int>(year / 100));
+      write2(static_cast<int>(year % 100));
+    } else {
+      write_year_extended(year);
+    }
+  }
+
+  void write_utc_offset(long offset) {
+    if (offset < 0) {
+      *out_++ = '-';
+      offset = -offset;
+    } else {
+      *out_++ = '+';
+    }
+    offset /= 60;
+    write2(static_cast<int>(offset / 60));
+    write2(static_cast<int>(offset % 60));
+  }
+  template <typename T, FMT_ENABLE_IF(has_member_data_tm_gmtoff<T>::value)>
+  void format_utc_offset_impl(const T& tm) {
+    write_utc_offset(tm.tm_gmtoff);
+  }
+  template <typename T, FMT_ENABLE_IF(!has_member_data_tm_gmtoff<T>::value)>
+  void format_utc_offset_impl(const T& tm) {
+#if defined(_WIN32) && defined(_UCRT)
+#  if FMT_USE_TZSET
+    tzset_once();
+#  endif
+    long offset = 0;
+    _get_timezone(&offset);
+    if (tm.tm_isdst) {
+      long dstbias = 0;
+      _get_dstbias(&dstbias);
+      offset += dstbias;
+    }
+    write_utc_offset(-offset);
+#else
+    ignore_unused(tm);
+    format_localized('z');
+#endif
+  }
+
+  template <typename T, FMT_ENABLE_IF(has_member_data_tm_zone<T>::value)>
+  void format_tz_name_impl(const T& tm) {
+    if (is_classic_)
+      out_ = write_tm_str<Char>(out_, tm.tm_zone, loc_);
+    else
+      format_localized('Z');
+  }
+  template <typename T, FMT_ENABLE_IF(!has_member_data_tm_zone<T>::value)>
+  void format_tz_name_impl(const T&) {
+    format_localized('Z');
+  }
+
+  void format_localized(char format, char modifier = 0) {
+    out_ = write<Char>(out_, tm_, loc_, format, modifier);
+  }
+
+ public:
+  tm_writer(const std::locale& loc, OutputIt out, const std::tm& tm)
+      : loc_(loc),
+        is_classic_(loc_ == get_classic_locale()),
+        out_(out),
+        tm_(tm) {}
+
+  OutputIt out() const { return out_; }
+
+  FMT_CONSTEXPR void on_text(const Char* begin, const Char* end) {
+    out_ = copy_str<Char>(begin, end, out_);
+  }
+
+  void on_abbr_weekday() {
+    if (is_classic_)
+      out_ = write(out_, tm_wday_short_name(tm_wday()));
+    else
+      format_localized('a');
+  }
+  void on_full_weekday() {
+    if (is_classic_)
+      out_ = write(out_, tm_wday_full_name(tm_wday()));
+    else
+      format_localized('A');
+  }
+  void on_dec0_weekday(numeric_system ns) {
+    if (is_classic_ || ns == numeric_system::standard) return write1(tm_wday());
+    format_localized('w', 'O');
+  }
+  void on_dec1_weekday(numeric_system ns) {
+    if (is_classic_ || ns == numeric_system::standard) {
+      auto wday = tm_wday();
+      write1(wday == 0 ? days_per_week : wday);
+    } else {
+      format_localized('u', 'O');
+    }
+  }
+
+  void on_abbr_month() {
+    if (is_classic_)
+      out_ = write(out_, tm_mon_short_name(tm_mon()));
+    else
+      format_localized('b');
+  }
+  void on_full_month() {
+    if (is_classic_)
+      out_ = write(out_, tm_mon_full_name(tm_mon()));
+    else
+      format_localized('B');
+  }
+
+  void on_datetime(numeric_system ns) {
+    if (is_classic_) {
+      on_abbr_weekday();
+      *out_++ = ' ';
+      on_abbr_month();
+      *out_++ = ' ';
+      on_day_of_month_space(numeric_system::standard);
+      *out_++ = ' ';
+      on_iso_time();
+      *out_++ = ' ';
+      on_year(numeric_system::standard);
+    } else {
+      format_localized('c', ns == numeric_system::standard ? '\0' : 'E');
+    }
+  }
+  void on_loc_date(numeric_system ns) {
+    if (is_classic_)
+      on_us_date();
+    else
+      format_localized('x', ns == numeric_system::standard ? '\0' : 'E');
+  }
+  void on_loc_time(numeric_system ns) {
+    if (is_classic_)
+      on_iso_time();
+    else
+      format_localized('X', ns == numeric_system::standard ? '\0' : 'E');
+  }
+  void on_us_date() {
+    char buf[8];
+    write_digit2_separated(buf, to_unsigned(tm_mon() + 1),
+                           to_unsigned(tm_mday()),
+                           to_unsigned(split_year_lower(tm_year())), '/');
+    out_ = copy_str<Char>(std::begin(buf), std::end(buf), out_);
+  }
+  void on_iso_date() {
+    auto year = tm_year();
+    char buf[10];
+    size_t offset = 0;
+    if (year >= 0 && year < 10000) {
+      copy2(buf, digits2(to_unsigned(year / 100)));
+    } else {
+      offset = 4;
+      write_year_extended(year);
+      year = 0;
+    }
+    write_digit2_separated(buf + 2, static_cast<unsigned>(year % 100),
+                           to_unsigned(tm_mon() + 1), to_unsigned(tm_mday()),
+                           '-');
+    out_ = copy_str<Char>(std::begin(buf) + offset, std::end(buf), out_);
+  }
+
+  void on_utc_offset() { format_utc_offset_impl(tm_); }
+  void on_tz_name() { format_tz_name_impl(tm_); }
+
+  void on_year(numeric_system ns) {
+    if (is_classic_ || ns == numeric_system::standard)
+      return write_year(tm_year());
+    format_localized('Y', 'E');
+  }
+  void on_short_year(numeric_system ns) {
+    if (is_classic_ || ns == numeric_system::standard)
+      return write2(split_year_lower(tm_year()));
+    format_localized('y', 'O');
+  }
+  void on_offset_year() {
+    if (is_classic_) return write2(split_year_lower(tm_year()));
+    format_localized('y', 'E');
+  }
+
+  void on_century(numeric_system ns) {
+    if (is_classic_ || ns == numeric_system::standard) {
+      auto year = tm_year();
+      auto upper = year / 100;
+      if (year >= -99 && year < 0) {
+        // Zero upper on negative year.
+        *out_++ = '-';
+        *out_++ = '0';
+      } else if (upper >= 0 && upper < 100) {
+        write2(static_cast<int>(upper));
+      } else {
+        out_ = write<Char>(out_, upper);
+      }
+    } else {
+      format_localized('C', 'E');
+    }
+  }
+
+  void on_dec_month(numeric_system ns) {
+    if (is_classic_ || ns == numeric_system::standard)
+      return write2(tm_mon() + 1);
+    format_localized('m', 'O');
+  }
+
+  void on_dec0_week_of_year(numeric_system ns) {
+    if (is_classic_ || ns == numeric_system::standard)
+      return write2((tm_yday() + days_per_week - tm_wday()) / days_per_week);
+    format_localized('U', 'O');
+  }
+  void on_dec1_week_of_year(numeric_system ns) {
+    if (is_classic_ || ns == numeric_system::standard) {
+      auto wday = tm_wday();
+      write2((tm_yday() + days_per_week -
+              (wday == 0 ? (days_per_week - 1) : (wday - 1))) /
+             days_per_week);
+    } else {
+      format_localized('W', 'O');
+    }
+  }
+  void on_iso_week_of_year(numeric_system ns) {
+    if (is_classic_ || ns == numeric_system::standard)
+      return write2(tm_iso_week_of_year());
+    format_localized('V', 'O');
+  }
+
+  void on_iso_week_based_year() { write_year(tm_iso_week_year()); }
+  void on_iso_week_based_short_year() {
+    write2(split_year_lower(tm_iso_week_year()));
+  }
+
+  void on_day_of_year() {
+    auto yday = tm_yday() + 1;
+    write1(yday / 100);
+    write2(yday % 100);
+  }
+  void on_day_of_month(numeric_system ns) {
+    if (is_classic_ || ns == numeric_system::standard) return write2(tm_mday());
+    format_localized('d', 'O');
+  }
+  void on_day_of_month_space(numeric_system ns) {
+    if (is_classic_ || ns == numeric_system::standard) {
+      auto mday = to_unsigned(tm_mday()) % 100;
+      const char* d2 = digits2(mday);
+      *out_++ = mday < 10 ? ' ' : d2[0];
+      *out_++ = d2[1];
+    } else {
+      format_localized('e', 'O');
+    }
+  }
+
+  void on_24_hour(numeric_system ns) {
+    if (is_classic_ || ns == numeric_system::standard) return write2(tm_hour());
+    format_localized('H', 'O');
+  }
+  void on_12_hour(numeric_system ns) {
+    if (is_classic_ || ns == numeric_system::standard)
+      return write2(tm_hour12());
+    format_localized('I', 'O');
+  }
+  void on_minute(numeric_system ns) {
+    if (is_classic_ || ns == numeric_system::standard) return write2(tm_min());
+    format_localized('M', 'O');
+  }
+  void on_second(numeric_system ns) {
+    if (is_classic_ || ns == numeric_system::standard) return write2(tm_sec());
+    format_localized('S', 'O');
+  }
+
+  void on_12_hour_time() {
+    if (is_classic_) {
+      char buf[8];
+      write_digit2_separated(buf, to_unsigned(tm_hour12()),
+                             to_unsigned(tm_min()), to_unsigned(tm_sec()), ':');
+      out_ = copy_str<Char>(std::begin(buf), std::end(buf), out_);
+      *out_++ = ' ';
+      on_am_pm();
+    } else {
+      format_localized('r');
+    }
+  }
+  void on_24_hour_time() {
+    write2(tm_hour());
+    *out_++ = ':';
+    write2(tm_min());
+  }
+  void on_iso_time() {
+    char buf[8];
+    write_digit2_separated(buf, to_unsigned(tm_hour()), to_unsigned(tm_min()),
+                           to_unsigned(tm_sec()), ':');
+    out_ = copy_str<Char>(std::begin(buf), std::end(buf), out_);
+  }
+
+  void on_am_pm() {
+    if (is_classic_) {
+      *out_++ = tm_hour() < 12 ? 'A' : 'P';
+      *out_++ = 'M';
+    } else {
+      format_localized('p');
+    }
+  }
+
+  // These apply to chrono durations but not tm.
+  void on_duration_value() {}
+  void on_duration_unit() {}
+};
+
+struct chrono_format_checker : null_chrono_spec_handler<chrono_format_checker> {
+  FMT_NORETURN void unsupported() { FMT_THROW(format_error("no date")); }
+
+  template <typename Char>
+  FMT_CONSTEXPR void on_text(const Char*, const Char*) {}
+  FMT_CONSTEXPR void on_24_hour(numeric_system) {}
+  FMT_CONSTEXPR void on_12_hour(numeric_system) {}
+  FMT_CONSTEXPR void on_minute(numeric_system) {}
+  FMT_CONSTEXPR void on_second(numeric_system) {}
+  FMT_CONSTEXPR void on_12_hour_time() {}
+  FMT_CONSTEXPR void on_24_hour_time() {}
+  FMT_CONSTEXPR void on_iso_time() {}
+  FMT_CONSTEXPR void on_am_pm() {}
+  FMT_CONSTEXPR void on_duration_value() {}
+  FMT_CONSTEXPR void on_duration_unit() {}
+};
+
+template <typename T, FMT_ENABLE_IF(std::is_integral<T>::value)>
+inline bool isnan(T) {
+  return false;
+}
+template <typename T, FMT_ENABLE_IF(std::is_floating_point<T>::value)>
+inline bool isnan(T value) {
+  return std::isnan(value);
+}
+
+template <typename T, FMT_ENABLE_IF(std::is_integral<T>::value)>
+inline bool isfinite(T) {
+  return true;
+}
+
+// Converts value to Int and checks that it's in the range [0, upper).
+template <typename T, typename Int, FMT_ENABLE_IF(std::is_integral<T>::value)>
+inline Int to_nonnegative_int(T value, Int upper) {
+  FMT_ASSERT(value >= 0 && to_unsigned(value) <= to_unsigned(upper),
+             "invalid value");
+  (void)upper;
+  return static_cast<Int>(value);
+}
+template <typename T, typename Int, FMT_ENABLE_IF(!std::is_integral<T>::value)>
+inline Int to_nonnegative_int(T value, Int upper) {
+  if (value < 0 || value > static_cast<T>(upper))
+    FMT_THROW(format_error("invalid value"));
+  return static_cast<Int>(value);
+}
+
+template <typename T, FMT_ENABLE_IF(std::is_integral<T>::value)>
+inline T mod(T x, int y) {
+  return x % static_cast<T>(y);
+}
+template <typename T, FMT_ENABLE_IF(std::is_floating_point<T>::value)>
+inline T mod(T x, int y) {
+  return std::fmod(x, static_cast<T>(y));
+}
+
+// If T is an integral type, maps T to its unsigned counterpart, otherwise
+// leaves it unchanged (unlike std::make_unsigned).
+template <typename T, bool INTEGRAL = std::is_integral<T>::value>
+struct make_unsigned_or_unchanged {
+  using type = T;
+};
+
+template <typename T> struct make_unsigned_or_unchanged<T, true> {
+  using type = typename std::make_unsigned<T>::type;
+};
+
+#if FMT_SAFE_DURATION_CAST
+// throwing version of safe_duration_cast
+template <typename To, typename FromRep, typename FromPeriod>
+To fmt_safe_duration_cast(std::chrono::duration<FromRep, FromPeriod> from) {
+  int ec;
+  To to = safe_duration_cast::safe_duration_cast<To>(from, ec);
+  if (ec) FMT_THROW(format_error("cannot format duration"));
+  return to;
+}
+#endif
+
+template <typename Rep, typename Period,
+          FMT_ENABLE_IF(std::is_integral<Rep>::value)>
+inline std::chrono::duration<Rep, std::milli> get_milliseconds(
+    std::chrono::duration<Rep, Period> d) {
+  // this may overflow and/or the result may not fit in the
+  // target type.
+#if FMT_SAFE_DURATION_CAST
+  using CommonSecondsType =
+      typename std::common_type<decltype(d), std::chrono::seconds>::type;
+  const auto d_as_common = fmt_safe_duration_cast<CommonSecondsType>(d);
+  const auto d_as_whole_seconds =
+      fmt_safe_duration_cast<std::chrono::seconds>(d_as_common);
+  // this conversion should be nonproblematic
+  const auto diff = d_as_common - d_as_whole_seconds;
+  const auto ms =
+      fmt_safe_duration_cast<std::chrono::duration<Rep, std::milli>>(diff);
+  return ms;
+#else
+  auto s = std::chrono::duration_cast<std::chrono::seconds>(d);
+  return std::chrono::duration_cast<std::chrono::milliseconds>(d - s);
+#endif
+}
+
+// Returns the number of fractional digits in the range [0, 18] according to the
+// C++20 spec. If more than 18 fractional digits are required then returns 6 for
+// microseconds precision.
+constexpr int count_fractional_digits(long long num, long long den, int n = 0) {
+  return num % den == 0
+             ? n
+             : (n > 18 ? 6 : count_fractional_digits(num * 10, den, n + 1));
+}
+
+constexpr long long pow10(std::uint32_t n) {
+  return n == 0 ? 1 : 10 * pow10(n - 1);
+}
+
+template <class Rep, class Period,
+          FMT_ENABLE_IF(std::numeric_limits<Rep>::is_signed)>
+constexpr std::chrono::duration<Rep, Period> abs(
+    std::chrono::duration<Rep, Period> d) {
+  // We need to compare the duration using the count() method directly
+  // due to a compiler bug in clang-11 regarding the spaceship operator,
+  // when -Wzero-as-null-pointer-constant is enabled.
+  // In clang-12 the bug has been fixed. See
+  // https://bugs.llvm.org/show_bug.cgi?id=46235 and the reproducible example:
+  // https://www.godbolt.org/z/Knbb5joYx.
+  return d.count() >= d.zero().count() ? d : -d;
+}
+
+template <class Rep, class Period,
+          FMT_ENABLE_IF(!std::numeric_limits<Rep>::is_signed)>
+constexpr std::chrono::duration<Rep, Period> abs(
+    std::chrono::duration<Rep, Period> d) {
+  return d;
+}
+
+template <typename Char, typename Rep, typename OutputIt,
+          FMT_ENABLE_IF(std::is_integral<Rep>::value)>
+OutputIt format_duration_value(OutputIt out, Rep val, int) {
+  return write<Char>(out, val);
+}
+
+template <typename Char, typename Rep, typename OutputIt,
+          FMT_ENABLE_IF(std::is_floating_point<Rep>::value)>
+OutputIt format_duration_value(OutputIt out, Rep val, int precision) {
+  auto specs = basic_format_specs<Char>();
+  specs.precision = precision;
+  specs.type = precision >= 0 ? presentation_type::fixed_lower
+                              : presentation_type::general_lower;
+  return write<Char>(out, val, specs);
+}
+
+template <typename Char, typename OutputIt>
+OutputIt copy_unit(string_view unit, OutputIt out, Char) {
+  return std::copy(unit.begin(), unit.end(), out);
+}
+
+template <typename OutputIt>
+OutputIt copy_unit(string_view unit, OutputIt out, wchar_t) {
+  // This works when wchar_t is UTF-32 because units only contain characters
+  // that have the same representation in UTF-16 and UTF-32.
+  utf8_to_utf16 u(unit);
+  return std::copy(u.c_str(), u.c_str() + u.size(), out);
+}
+
+template <typename Char, typename Period, typename OutputIt>
+OutputIt format_duration_unit(OutputIt out) {
+  if (const char* unit = get_units<Period>())
+    return copy_unit(string_view(unit), out, Char());
+  *out++ = '[';
+  out = write<Char>(out, Period::num);
+  if (const_check(Period::den != 1)) {
+    *out++ = '/';
+    out = write<Char>(out, Period::den);
+  }
+  *out++ = ']';
+  *out++ = 's';
+  return out;
+}
+
+class get_locale {
+ private:
+  union {
+    std::locale locale_;
+  };
+  bool has_locale_ = false;
+
+ public:
+  get_locale(bool localized, locale_ref loc) : has_locale_(localized) {
+    if (localized)
+      ::new (&locale_) std::locale(loc.template get<std::locale>());
+  }
+  ~get_locale() {
+    if (has_locale_) locale_.~locale();
+  }
+  operator const std::locale&() const {
+    return has_locale_ ? locale_ : get_classic_locale();
+  }
+};
+
+template <typename FormatContext, typename OutputIt, typename Rep,
+          typename Period>
+struct chrono_formatter {
+  FormatContext& context;
+  OutputIt out;
+  int precision;
+  bool localized = false;
+  // rep is unsigned to avoid overflow.
+  using rep =
+      conditional_t<std::is_integral<Rep>::value && sizeof(Rep) < sizeof(int),
+                    unsigned, typename make_unsigned_or_unchanged<Rep>::type>;
+  rep val;
+  using seconds = std::chrono::duration<rep>;
+  seconds s;
+  using milliseconds = std::chrono::duration<rep, std::milli>;
+  bool negative;
+
+  using char_type = typename FormatContext::char_type;
+  using tm_writer_type = tm_writer<OutputIt, char_type>;
+
+  chrono_formatter(FormatContext& ctx, OutputIt o,
+                   std::chrono::duration<Rep, Period> d)
+      : context(ctx),
+        out(o),
+        val(static_cast<rep>(d.count())),
+        negative(false) {
+    if (d.count() < 0) {
+      val = 0 - val;
+      negative = true;
+    }
+
+    // this may overflow and/or the result may not fit in the
+    // target type.
+#if FMT_SAFE_DURATION_CAST
+    // might need checked conversion (rep!=Rep)
+    auto tmpval = std::chrono::duration<rep, Period>(val);
+    s = fmt_safe_duration_cast<seconds>(tmpval);
+#else
+    s = std::chrono::duration_cast<seconds>(
+        std::chrono::duration<rep, Period>(val));
+#endif
+  }
+
+  // returns true if nan or inf, writes to out.
+  bool handle_nan_inf() {
+    if (isfinite(val)) {
+      return false;
+    }
+    if (isnan(val)) {
+      write_nan();
+      return true;
+    }
+    // must be +-inf
+    if (val > 0) {
+      write_pinf();
+    } else {
+      write_ninf();
+    }
+    return true;
+  }
+
+  Rep hour() const { return static_cast<Rep>(mod((s.count() / 3600), 24)); }
+
+  Rep hour12() const {
+    Rep hour = static_cast<Rep>(mod((s.count() / 3600), 12));
+    return hour <= 0 ? 12 : hour;
+  }
+
+  Rep minute() const { return static_cast<Rep>(mod((s.count() / 60), 60)); }
+  Rep second() const { return static_cast<Rep>(mod(s.count(), 60)); }
+
+  std::tm time() const {
+    auto time = std::tm();
+    time.tm_hour = to_nonnegative_int(hour(), 24);
+    time.tm_min = to_nonnegative_int(minute(), 60);
+    time.tm_sec = to_nonnegative_int(second(), 60);
+    return time;
+  }
+
+  void write_sign() {
+    if (negative) {
+      *out++ = '-';
+      negative = false;
+    }
+  }
+
+  void write(Rep value, int width) {
+    write_sign();
+    if (isnan(value)) return write_nan();
+    uint32_or_64_or_128_t<int> n =
+        to_unsigned(to_nonnegative_int(value, max_value<int>()));
+    int num_digits = detail::count_digits(n);
+    if (width > num_digits) out = std::fill_n(out, width - num_digits, '0');
+    out = format_decimal<char_type>(out, n, num_digits).end;
+  }
+
+  template <class Duration> void write_fractional_seconds(Duration d) {
+    constexpr auto num_fractional_digits =
+        count_fractional_digits(Duration::period::num, Duration::period::den);
+
+    using subsecond_precision = std::chrono::duration<
+        typename std::common_type<typename Duration::rep,
+                                  std::chrono::seconds::rep>::type,
+        std::ratio<1, detail::pow10(num_fractional_digits)>>;
+    if (std::ratio_less<typename subsecond_precision::period,
+                        std::chrono::seconds::period>::value) {
+      *out++ = '.';
+      // Don't convert long double to integer seconds to avoid overflow.
+      using sec = conditional_t<
+          std::is_same<typename Duration::rep, long double>::value,
+          std::chrono::duration<long double>, std::chrono::seconds>;
+      auto fractional = detail::abs(d) - std::chrono::duration_cast<sec>(d);
+      const auto subseconds =
+          std::chrono::treat_as_floating_point<
+              typename subsecond_precision::rep>::value
+              ? fractional.count()
+              : std::chrono::duration_cast<subsecond_precision>(fractional)
+                    .count();
+      uint32_or_64_or_128_t<long long> n =
+          to_unsigned(to_nonnegative_int(subseconds, max_value<long long>()));
+      int num_digits = detail::count_digits(n);
+      if (num_fractional_digits > num_digits)
+        out = std::fill_n(out, num_fractional_digits - num_digits, '0');
+      out = format_decimal<char_type>(out, n, num_digits).end;
+    }
+  }
+
+  void write_nan() { std::copy_n("nan", 3, out); }
+  void write_pinf() { std::copy_n("inf", 3, out); }
+  void write_ninf() { std::copy_n("-inf", 4, out); }
+
+  template <typename Callback, typename... Args>
+  void format_tm(const tm& time, Callback cb, Args... args) {
+    if (isnan(val)) return write_nan();
+    get_locale loc(localized, context.locale());
+    auto w = tm_writer_type(loc, out, time);
+    (w.*cb)(args...);
+    out = w.out();
+  }
+
+  void on_text(const char_type* begin, const char_type* end) {
+    std::copy(begin, end, out);
+  }
+
+  // These are not implemented because durations don't have date information.
+  void on_abbr_weekday() {}
+  void on_full_weekday() {}
+  void on_dec0_weekday(numeric_system) {}
+  void on_dec1_weekday(numeric_system) {}
+  void on_abbr_month() {}
+  void on_full_month() {}
+  void on_datetime(numeric_system) {}
+  void on_loc_date(numeric_system) {}
+  void on_loc_time(numeric_system) {}
+  void on_us_date() {}
+  void on_iso_date() {}
+  void on_utc_offset() {}
+  void on_tz_name() {}
+  void on_year(numeric_system) {}
+  void on_short_year(numeric_system) {}
+  void on_offset_year() {}
+  void on_century(numeric_system) {}
+  void on_iso_week_based_year() {}
+  void on_iso_week_based_short_year() {}
+  void on_dec_month(numeric_system) {}
+  void on_dec0_week_of_year(numeric_system) {}
+  void on_dec1_week_of_year(numeric_system) {}
+  void on_iso_week_of_year(numeric_system) {}
+  void on_day_of_year() {}
+  void on_day_of_month(numeric_system) {}
+  void on_day_of_month_space(numeric_system) {}
+
+  void on_24_hour(numeric_system ns) {
+    if (handle_nan_inf()) return;
+
+    if (ns == numeric_system::standard) return write(hour(), 2);
+    auto time = tm();
+    time.tm_hour = to_nonnegative_int(hour(), 24);
+    format_tm(time, &tm_writer_type::on_24_hour, ns);
+  }
+
+  void on_12_hour(numeric_system ns) {
+    if (handle_nan_inf()) return;
+
+    if (ns == numeric_system::standard) return write(hour12(), 2);
+    auto time = tm();
+    time.tm_hour = to_nonnegative_int(hour12(), 12);
+    format_tm(time, &tm_writer_type::on_12_hour, ns);
+  }
+
+  void on_minute(numeric_system ns) {
+    if (handle_nan_inf()) return;
+
+    if (ns == numeric_system::standard) return write(minute(), 2);
+    auto time = tm();
+    time.tm_min = to_nonnegative_int(minute(), 60);
+    format_tm(time, &tm_writer_type::on_minute, ns);
+  }
+
+  void on_second(numeric_system ns) {
+    if (handle_nan_inf()) return;
+
+    if (ns == numeric_system::standard) {
+      write(second(), 2);
+      write_fractional_seconds(std::chrono::duration<rep, Period>{val});
+      return;
+    }
+    auto time = tm();
+    time.tm_sec = to_nonnegative_int(second(), 60);
+    format_tm(time, &tm_writer_type::on_second, ns);
+  }
+
+  void on_12_hour_time() {
+    if (handle_nan_inf()) return;
+    format_tm(time(), &tm_writer_type::on_12_hour_time);
+  }
+
+  void on_24_hour_time() {
+    if (handle_nan_inf()) {
+      *out++ = ':';
+      handle_nan_inf();
+      return;
+    }
+
+    write(hour(), 2);
+    *out++ = ':';
+    write(minute(), 2);
+  }
+
+  void on_iso_time() {
+    on_24_hour_time();
+    *out++ = ':';
+    if (handle_nan_inf()) return;
+    on_second(numeric_system::standard);
+  }
+
+  void on_am_pm() {
+    if (handle_nan_inf()) return;
+    format_tm(time(), &tm_writer_type::on_am_pm);
+  }
+
+  void on_duration_value() {
+    if (handle_nan_inf()) return;
+    write_sign();
+    out = format_duration_value<char_type>(out, val, precision);
+  }
+
+  void on_duration_unit() {
+    out = format_duration_unit<char_type, Period>(out);
+  }
+};
+
+FMT_END_DETAIL_NAMESPACE
+
+#if defined(__cpp_lib_chrono) && __cpp_lib_chrono >= 201907
+using weekday = std::chrono::weekday;
+#else
+// A fallback version of weekday.
+class weekday {
+ private:
+  unsigned char value;
+
+ public:
+  weekday() = default;
+  explicit constexpr weekday(unsigned wd) noexcept
+      : value(static_cast<unsigned char>(wd != 7 ? wd : 0)) {}
+  constexpr unsigned c_encoding() const noexcept { return value; }
+};
+
+class year_month_day {};
+#endif
+
+// A rudimentary weekday formatter.
+template <typename Char> struct formatter<weekday, Char> {
+ private:
+  bool localized = false;
+
+ public:
+  FMT_CONSTEXPR auto parse(basic_format_parse_context<Char>& ctx)
+      -> decltype(ctx.begin()) {
+    auto begin = ctx.begin(), end = ctx.end();
+    if (begin != end && *begin == 'L') {
+      ++begin;
+      localized = true;
+    }
+    return begin;
+  }
+
+  template <typename FormatContext>
+  auto format(weekday wd, FormatContext& ctx) const -> decltype(ctx.out()) {
+    auto time = std::tm();
+    time.tm_wday = static_cast<int>(wd.c_encoding());
+    detail::get_locale loc(localized, ctx.locale());
+    auto w = detail::tm_writer<decltype(ctx.out()), Char>(loc, ctx.out(), time);
+    w.on_abbr_weekday();
+    return w.out();
+  }
+};
+
+template <typename Rep, typename Period, typename Char>
+struct formatter<std::chrono::duration<Rep, Period>, Char> {
+ private:
+  basic_format_specs<Char> specs;
+  int precision = -1;
+  using arg_ref_type = detail::arg_ref<Char>;
+  arg_ref_type width_ref;
+  arg_ref_type precision_ref;
+  bool localized = false;
+  basic_string_view<Char> format_str;
+  using duration = std::chrono::duration<Rep, Period>;
+
+  struct spec_handler {
+    formatter& f;
+    basic_format_parse_context<Char>& context;
+    basic_string_view<Char> format_str;
+
+    template <typename Id> FMT_CONSTEXPR arg_ref_type make_arg_ref(Id arg_id) {
+      context.check_arg_id(arg_id);
+      return arg_ref_type(arg_id);
+    }
+
+    FMT_CONSTEXPR arg_ref_type make_arg_ref(basic_string_view<Char> arg_id) {
+      context.check_arg_id(arg_id);
+      return arg_ref_type(arg_id);
+    }
+
+    FMT_CONSTEXPR arg_ref_type make_arg_ref(detail::auto_id) {
+      return arg_ref_type(context.next_arg_id());
+    }
+
+    void on_error(const char* msg) { FMT_THROW(format_error(msg)); }
+    FMT_CONSTEXPR void on_fill(basic_string_view<Char> fill) {
+      f.specs.fill = fill;
+    }
+    FMT_CONSTEXPR void on_align(align_t align) { f.specs.align = align; }
+    FMT_CONSTEXPR void on_width(int width) { f.specs.width = width; }
+    FMT_CONSTEXPR void on_precision(int _precision) {
+      f.precision = _precision;
+    }
+    FMT_CONSTEXPR void end_precision() {}
+
+    template <typename Id> FMT_CONSTEXPR void on_dynamic_width(Id arg_id) {
+      f.width_ref = make_arg_ref(arg_id);
+    }
+
+    template <typename Id> FMT_CONSTEXPR void on_dynamic_precision(Id arg_id) {
+      f.precision_ref = make_arg_ref(arg_id);
+    }
+  };
+
+  using iterator = typename basic_format_parse_context<Char>::iterator;
+  struct parse_range {
+    iterator begin;
+    iterator end;
+  };
+
+  FMT_CONSTEXPR parse_range do_parse(basic_format_parse_context<Char>& ctx) {
+    auto begin = ctx.begin(), end = ctx.end();
+    if (begin == end || *begin == '}') return {begin, begin};
+    spec_handler handler{*this, ctx, format_str};
+    begin = detail::parse_align(begin, end, handler);
+    if (begin == end) return {begin, begin};
+    begin = detail::parse_width(begin, end, handler);
+    if (begin == end) return {begin, begin};
+    if (*begin == '.') {
+      if (std::is_floating_point<Rep>::value)
+        begin = detail::parse_precision(begin, end, handler);
+      else
+        handler.on_error("precision not allowed for this argument type");
+    }
+    if (begin != end && *begin == 'L') {
+      ++begin;
+      localized = true;
+    }
+    end = detail::parse_chrono_format(begin, end,
+                                      detail::chrono_format_checker());
+    return {begin, end};
+  }
+
+ public:
+  FMT_CONSTEXPR auto parse(basic_format_parse_context<Char>& ctx)
+      -> decltype(ctx.begin()) {
+    auto range = do_parse(ctx);
+    format_str = basic_string_view<Char>(
+        &*range.begin, detail::to_unsigned(range.end - range.begin));
+    return range.end;
+  }
+
+  template <typename FormatContext>
+  auto format(const duration& d, FormatContext& ctx) const
+      -> decltype(ctx.out()) {
+    auto specs_copy = specs;
+    auto precision_copy = precision;
+    auto begin = format_str.begin(), end = format_str.end();
+    // As a possible future optimization, we could avoid extra copying if width
+    // is not specified.
+    basic_memory_buffer<Char> buf;
+    auto out = std::back_inserter(buf);
+    detail::handle_dynamic_spec<detail::width_checker>(specs_copy.width,
+                                                       width_ref, ctx);
+    detail::handle_dynamic_spec<detail::precision_checker>(precision_copy,
+                                                           precision_ref, ctx);
+    if (begin == end || *begin == '}') {
+      out = detail::format_duration_value<Char>(out, d.count(), precision_copy);
+      detail::format_duration_unit<Char, Period>(out);
+    } else {
+      detail::chrono_formatter<FormatContext, decltype(out), Rep, Period> f(
+          ctx, out, d);
+      f.precision = precision_copy;
+      f.localized = localized;
+      detail::parse_chrono_format(begin, end, f);
+    }
+    return detail::write(
+        ctx.out(), basic_string_view<Char>(buf.data(), buf.size()), specs_copy);
+  }
+};
+
+template <typename Char, typename Duration>
+struct formatter<std::chrono::time_point<std::chrono::system_clock, Duration>,
+                 Char> : formatter<std::tm, Char> {
+  FMT_CONSTEXPR formatter() {
+    this->do_parse(default_specs,
+                   default_specs + sizeof(default_specs) / sizeof(Char));
+  }
+
+  template <typename ParseContext>
+  FMT_CONSTEXPR auto parse(ParseContext& ctx) -> decltype(ctx.begin()) {
+    return this->do_parse(ctx.begin(), ctx.end(), true);
+  }
+
+  template <typename FormatContext>
+  auto format(std::chrono::time_point<std::chrono::system_clock> val,
+              FormatContext& ctx) const -> decltype(ctx.out()) {
+    return formatter<std::tm, Char>::format(localtime(val), ctx);
+  }
+
+  static constexpr const Char default_specs[] = {'%', 'F', ' ', '%', 'T'};
+};
+
+template <typename Char, typename Duration>
+constexpr const Char
+    formatter<std::chrono::time_point<std::chrono::system_clock, Duration>,
+              Char>::default_specs[];
+
+template <typename Char> struct formatter<std::tm, Char> {
+ private:
+  enum class spec {
+    unknown,
+    year_month_day,
+    hh_mm_ss,
+  };
+  spec spec_ = spec::unknown;
+  basic_string_view<Char> specs;
+
+ protected:
+  template <typename It>
+  FMT_CONSTEXPR auto do_parse(It begin, It end, bool with_default = false)
+      -> It {
+    if (begin != end && *begin == ':') ++begin;
+    end = detail::parse_chrono_format(begin, end, detail::tm_format_checker());
+    if (!with_default || end != begin)
+      specs = {begin, detail::to_unsigned(end - begin)};
+    // basic_string_view<>::compare isn't constexpr before C++17.
+    if (specs.size() == 2 && specs[0] == Char('%')) {
+      if (specs[1] == Char('F'))
+        spec_ = spec::year_month_day;
+      else if (specs[1] == Char('T'))
+        spec_ = spec::hh_mm_ss;
+    }
+    return end;
+  }
+
+ public:
+  template <typename ParseContext>
+  FMT_CONSTEXPR auto parse(ParseContext& ctx) -> decltype(ctx.begin()) {
+    return this->do_parse(ctx.begin(), ctx.end());
+  }
+
+  template <typename FormatContext>
+  auto format(const std::tm& tm, FormatContext& ctx) const
+      -> decltype(ctx.out()) {
+    const auto loc_ref = ctx.locale();
+    detail::get_locale loc(static_cast<bool>(loc_ref), loc_ref);
+    auto w = detail::tm_writer<decltype(ctx.out()), Char>(loc, ctx.out(), tm);
+    if (spec_ == spec::year_month_day)
+      w.on_iso_date();
+    else if (spec_ == spec::hh_mm_ss)
+      w.on_iso_time();
+    else
+      detail::parse_chrono_format(specs.begin(), specs.end(), w);
+    return w.out();
+  }
+};
+
+FMT_MODULE_EXPORT_END
+FMT_END_NAMESPACE
+
+#endif  // FMT_CHRONO_H_