Set the timer's expiry time as an absolute time.
std::size_t expires_at( const time_point & expiry_time);
This function sets the expiry time. Any pending asynchronous wait operations
will be cancelled. The handler for each cancelled operation will be invoked
boost::asio::error::operation_aborted error code.
The expiry time to be used for the timer.
The number of asynchronous operations that were cancelled.
Thrown on failure.
If the timer has already expired when
expires_at() is called,
then the handlers for asynchronous wait operations will:
These handlers can no longer be cancelled, and therefore are passed an error code that indicates the successful completion of the wait operation.