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Constructor to set a particular expiry time as an absolute time.
basic_waitable_timer( asio::io_service & io_service, const time_point & expiry_time);
This constructor creates a timer and sets the expiry time.
                  The io_service
                  object that the timer will use 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.