25 Time library [time]

25.7 Clocks [time.clock]

25.7.9 time_­point conversions [time.clock.cast] Class template clock_­time_­conversion [time.clock.conv]

namespace std::chrono {
  template<class DestClock, class SourceClock>
  struct clock_time_conversion {};
clock_­time_­conversion serves as a trait which can be used to specify how to convert a source time_­point of type time_­point<SourceClock, Duration> to a destination time_­point of type time_­point<DestClock, Duration> via a specialization: clock_­time_­conversion<DestClock, SourceClock>.
A specialization of clock_­time_­conversion<DestClock, SourceClock> shall provide a const-qualified operator() that takes a parameter of type time_­point<SourceClock, Duration> and returns a time_­point<DestClock, OtherDuration> representing an equivalent point in time.
OtherDuration is a chrono::duration whose specialization is computed from the input Duration in a manner which can vary for each clock_­time_­conversion specialization.
A program may specialize clock_­time_­conversion if at least one of the template parameters is a user-defined clock type.
Several specializations are provided by the implementation, as described in [time.clock.cast.id], [time.clock.cast.sys.utc], [time.clock.cast.sys], and [time.clock.cast.utc].