def test_timePasses(self):
        """
        If a delayed call is scheduled and then some time passes, the
        timeout passed to C{doIteration} is reduced by the amount of time
        which passed.
        """
        reactor = TimeoutReportReactor()
        reactor.callLater(100, lambda: None)
        reactor.now += 25
        timeout = self._checkIterationTimeout(reactor)
        self.assertEqual(timeout, 75)

        If a delayed call is scheduled and then some time passes, the
        timeout passed to C{doIteration} is reduced by the amount of time
        which passed.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/internet/test/test_posixbase.py