Effects: Constructs an object of type time_point, initializing d_ with duration::zero(). Such a time_point object represents the epoch.
time_point(const duration& d);
Effects: Constructs an object of type time_point, initializing d_ with d. Such a time_point object represents the epoch + d.
template <class Duration2>
time_point(const time_point<clock, Duration2>& t);
Remarks: This constructor shall not participate in overload resolution unless Duration2 is implicitly convertible to duration.
Effects: Constructs an object of type time_point, initializing d_ with t.time_since_epoch().