Example:

   # fire an event after 7.7 seconds
   my $w = AnyEvent->timer (after => 7.7, cb => sub {
      warn "timeout\n";
   });

   # to cancel the timer:
   undef $w;

Example 2:

   # fire an event after 0.5 seconds, then roughly every second
   my $w;

   my $cb = sub {
      # cancel the old timer while creating a new one
      $w = AnyEvent->timer (after => 1, cb => $cb);
   };

   # start the "loop" by creating the first watcher
   $w = AnyEvent->timer (after => 0.5, cb => $cb);

=head3 TIMING ISSUES

There are two ways to handle timers: based on real time (relative, "fire
in 10 seconds") and based on wallclock time (absolute, "fire at 12
o'clock").

While most event loops expect timers to specified in a relative way, they
use absolute time internally. This makes a difference when your clock
"jumps", for example, when ntp decides to set your clock backwards from
the wrong date of 2014-01-01 to 2008-01-01, a watcher that is supposed to
fire "after" a second might actually take six years to finally fire.

AnyEvent cannot compensate for this. The only event loop that is conscious
about these issues is L<EV>, which offers both relative (ev_timer, based
on true relative time) and absolute (ev_periodic, based on wallclock time)
timers.

AnyEvent always prefers relative timers, if available, matching the
AnyEvent API.

AnyEvent has two additional methods that return the "current time":

=over 4

=item AnyEvent->time

This returns the "current wallclock time" as a fractional number of
seconds since the Epoch (the same thing as C<time> or C<Time::HiRes::time>
return, and the result is guaranteed to be compatible with those).

It progresses independently of any event loop processing, i.e. each call
will check the system clock, which usually gets updated frequently.

=item AnyEvent->now

This also returns the "current wallclock time", but unlike C<time>, above,
this value might change only once per event loop iteration, depending on
the event loop (most return the same time as C<time>, above). This is the
time that AnyEvent's timers get scheduled against.

I<In almost all cases (in all cases if you don't care), this is the
function to call when you want to know the current time.>

This function is also often faster then C<< AnyEvent->time >>, and
thus the preferred method if you want some timestamp (for example,
L<AnyEvent::Handle> uses this to update it's activity timeouts).

The rest of this section is only of relevance if you try to be very exact
with your timing, you can skip it without bad conscience.

For a practical example of when these times differ, consider L<Event::Lib>
and L<EV> and the following set-up:

The event loop is running and has just invoked one of your callback at
time=500 (assume no other callbacks delay processing). In your callback,
you wait a second by executing C<sleep 1> (blocking the process for a
second) and then (at time=501) you create a relative timer that fires
after three seconds.

With L<Event::Lib>, C<< AnyEvent->time >> and C<< AnyEvent->now >> will
both return C<501>, because that is the current time, and the timer will
be scheduled to fire at time=504 (C<501> + C<3>).

With L<EV>, C<< AnyEvent->time >> returns C<501> (as that is the current
time), but C<< AnyEvent->now >> returns C<500>, as that is the time the
last event processing phase started. With L<EV>, your timer gets scheduled
to run at time=503 (C<500> + C<3>).

In one sense, L<Event::Lib> is more exact, as it uses the current time
regardless of any delays introduced by event processing. However, most
callbacks do not expect large delays in processing, so this causes a
higher drift (and a lot more system calls to get the current time).

In another sense, L<EV> is more exact, as your timer will be scheduled at
the same time, regardless of how long event processing actually took.

In either case, if you care (and in most cases, you don't), then you