### Timers

Long running I/O operations will often have a deadline by which they must have completed. These deadlines may be expressed as absolute times, but are often calculated relative to the current time.

As a simple example, to perform a synchronous wait operation on a timer using a relative time one may write:

    io_context i;
    ...
    deadline_timer t(i);
    t.expires_from_now(boost::posix_time::seconds(5));
    t.wait();

More commonly, a program will perform an asynchronous wait operation on a timer:

    void handler(asio::error_code ec) { ... }
    ...
    io_context i;
    ...
    deadline_timer t(i);
    t.expires_from_now(boost::posix_time::milliseconds(400));
    t.async_wait(handler);
    ...
    i.run();

The deadline associated with a timer may also be obtained as a relative time:

    boost::posix_time::time_duration time_until_expiry
      = t.expires_from_now();

or as an absolute time to allow composition of timers:

    deadline_timer t2(i);
    t2.expires_at(t.expires_at() + boost::posix_time::seconds(30));

##### See Also

basic_deadline_timer, deadline_timer, timer tutorials.