[time.point.cons]

constexpr time_point();

Effects: Initializes d_ with duration::zero().

Such a time_point object represents the epoch.

constexpr explicit time_point(const duration& d);

Effects: Initializes d_ with d.

Such a time_point object represents the epoch + d.

template<class Duration2>
  constexpr time_point(const time_point<clock, Duration2>& t);

Effects: Initializes d_ with t.time_since_epoch().

30 Time library [time]