Set the timer's expiry time as an absolute time.
std::size_t expires_at( const time_type & 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 with the 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.