[time.12]

constexpr bool is_am(const hours& h) noexcept;

Returns: 0h <= h && h <= 11h.

constexpr bool is_pm(const hours& h) noexcept;

Returns: 12h <= h && h <= 23h.

constexpr hours make12(const hours& h) noexcept;

Returns: The 12-hour equivalent of h in the range [1h, 12h].

If h is not in the range [0h, 23h], the value returned is unspecified.

constexpr hours make24(const hours& h, bool is_pm) noexcept;

Returns: If is_pm is false, returns the 24-hour equivalent of h in the range [0h, 11h], assuming h represents an ante meridiem hour.

Otherwise, returns the 24-hour equivalent of h in the range [12h, 23h], assuming h represents a post meridiem hour.

If h is not in the range [1h, 12h], the value returned is unspecified.