std::chrono::is_am,std::chrono::is_pm,std::chrono::make12,std::chrono::make24(3)

NAME
std::chrono::is_am,std::chrono::is_pm,std::chrono::make12,std::chrono::make24 - std::chrono::is_am,std::chrono::is_pm,std::chrono::make12,std::chrono::make24

Synopsis
   Defined in header <chrono>
   constexpr bool is_am( const std::chrono::hours& h ) noexcept; (1) (since C++20)
   constexpr bool is_pm( const std::chrono::hours& h ) noexcept; (2) (since C++20)
   constexpr std::chrono::hours make12( const std::chrono::hours& h ) (3) (since C++20)
   noexcept;
   constexpr std::chrono::hours make24( const std::chrono::hours& h, (4) (since C++20)
                        bool is_pm ) noexcept;
   These functions aid in translating between a 12-hour format time of day, and a
   24-hour format time of day.
   1) Detects whether the 24-hour format time is a.m. (ante meridiem, before midday).
   2) Detects whether the 24-hour format time is p.m. (post meridiem, after midday).
   3) Returns the 12-hour equivalent of a 24-hour format time.
   4) Returns the 24-hour equivalent of a 12-hour format time h, where is_pm determines
   whether the time is p.m.

Parameters
   h - 12-hour or 24-hour format time to detect
   is_pm - whether the 12-hour format time is p.m.

Return value
   1) 0h <= h && h <= 11h
   2) 12h <= h && h <= 23h
   3) If h is in range [0h, 23h], returns the 12-hour equivalent in range [1h, 12h].
   Otherwise, the return value is unspecified.
   4) If h is in range [1h, 12h], returns the 24-hour equivalent in range [0h, 11h] if
   is_pm is false, or in range [12h, 23h] otherwise. Otherwise, the return value is
   unspecified.

Example
// Run this code
 #include <chrono>
 #include <iomanip>
 #include <iostream>
 #include <utility>
 int main()
 {
     using namespace std::chrono;
     static_assert(
         is_am(10h) && is_am(11h) && !is_am(12h) && !is_am(23h) &&
        !is_pm(10h) && !is_pm(11h) && is_pm(12h) && is_pm(23h)
     );
     std::cout << "make12():\n";
     for (const hours hh : {0h, 1h, 11h, 12h, 13h, 23h})
     {
         const hours am{make12(hh)};
         std::cout << std::setw(2) << hh.count() << "h == "
                   << std::setw(2) << am.count() << (is_am(hh) ? "h a.m.\n" : "h p.m.\n");
     }
     std::cout << "\nmake24():\n";
     using p = std::pair<hours, bool>;
     for (const auto& [hh, pm] : {p{1h, 0}, p{12h, 0}, p{1h, 1}, p{12h, 1}})
     {
         std::cout << std::setw(2) << hh.count()
                   << (pm ? "h p.m." : "h a.m.")
                   << " == " << std::setw(2)
                   << make24(hh, pm).count() << "h\n";
     }
 }

Output:
 make12():
  0h == 12h a.m.
  1h == 1h a.m.
 11h == 11h a.m.
 12h == 12h p.m.
 13h == 1h p.m.
 23h == 11h p.m.
 make24():
  1h a.m. == 1h
 12h a.m. == 0h
  1h p.m. == 13h
 12h p.m. == 12h