Constructor to set a particular expiry time relative to now.
template< typename ExecutionContext> basic_waitable_timer( ExecutionContext & context, const duration & expiry_time, typename enable_if< is_convertible< ExecutionContext &, execution_context & >::value >::type * = 0);
This constructor creates a timer and sets the expiry time.
An execution context which provides the I/O executor 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, relative to now.