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

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is_am is_pm
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make12 make24
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C-style date and time
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 ) noexcept ;
(3) (since C++20)
constexpr std:: chrono :: hours make24 ( const std:: chrono :: hours & h,
bool is_pm ) noexcept ;
(4) (since C++20)

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
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