Deferred equivalent of sleep?

waitfor.py

def waitFor(clock, delay, deferred):
    """
    Usage::

        waitFor(reactor, 20, d).addCallback(print)

    Will wait for 20s before calling ``print`` with the result of ``d``.
    """
    # H/T @tomprince
    return deferred.addCallback(lambda x: deferLater(clock, delay, lambda: x))