29 Time library [time]

29.5 Class template duration [time.duration]

29.5.10 Algorithms [time.duration.alg]

template<class Rep, class Period> constexpr duration<Rep, Period> abs(duration<Rep, Period> d);
Constraints: numeric_­limits<Rep>​::​is_­signed is true.
Returns: If d >= d.zero(), return d, otherwise return -d.