Constructor to set a particular expiry time as an absolute time.
basic_waitable_timer( const executor_type & ex, const time_point & expiry_time);
This constructor creates a timer and sets the expiry time.
The I/O executor object that the timer will use, by default, to dispatch handlers for any asynchronous operations performed on the timer.
The expiry time to be used for the timer, expressed as an absolute time.