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

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< cpp‎ | chrono

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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