'Runs the ``callback`` at the absolute time designated by ``when``.
``when`` must be a number using the same reference point as
`IOLoop.time`.
Returns an opaque handle that may be passed to `remove_timeout`
to cancel.  Note that unlike the `asyncio` method of the same
name, the returned object does not have a ``cancel()`` method.
See `add_timeout` for comments on thread-safety and subclassing.
.. versionadded:: 4.0'

def call_at(self, when, callback, *args, **kwargs):

    return self.add_timeout(when, callback, *args, **kwargs)
