Mock is not effective after twisted deferred is returned

I am having trouble with mocking in twisted trial unit test. The function under test returns a deferred, and I return that deferred from the unit test. However, it seems like the patches that I have applied are reverted once I return the deferred, it doesn't wait until the deferred is resolved.

Please see the following example:

schedule.py

import time
from twisted.internet import task, reactor

class Schedule():
    def get_time(self):
        t = time.time()
        return task.deferLater(reactor, 2, lambda: t)

    def get_time_future(self):
        return task.deferLater(reactor, 2, lambda: time.time())

schedule_test.py

import mock
from twisted.trial import unittest
from schedule import Schedule

class ScheduleTest(unittest.TestCase):
    @mock.patch('schedule.time')
    def test_get_time(self, mock_time):
        mock_time.time.return_value = 1450407660
        d = Schedule().get_time()
        d.addCallback(self.assertEqual, 1450407660)
        return d

    @mock.patch('schedule.time')
    def test_get_time_future(self, mock_time):
        mock_time.time.return_value = 1450407660
        d = Schedule().get_time_future()
        d.addCallback(self.assertEqual, 1450407660)
        return d

And the output

test_schedule
ScheduleTest
    test_get_time ...                                                      [OK]
    test_get_time_future ...                                             [FAIL]

===============================================================================
[FAIL]
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/twisted/trial/_synctest.py", line 437, in assertEqual
    super(_Assertions, self).assertEqual(first, second, msg)
  File "/usr/lib/python2.7/unittest/case.py", line 515, in assertEqual
    assertion_func(first, second, msg=msg)
  File "/usr/lib/python2.7/unittest/case.py", line 508, in _baseAssertEqual
    raise self.failureException(msg)
twisted.trial.unittest.FailTest: 1456370638.190432 != 1450407660

test_schedule.ScheduleTest.test_get_time_future
-------------------------------------------------------------------------------
Ran 2 tests in 4.024s

FAILED (failures=1, successes=1)

Is there something wrong in the way that I am mocking time in the above example?