import unittest
import datetime
from mock import patch

class SomeTestCase(unittest.TestCase):
    def setUp(self):
        self.time = datetime.datetime(2012, 5, 18)
        class fakedatetime(datetime.datetime):
            @classmethod
            def now(cls):
                return self.time
        patcher = patch('datetime.datetime', fakedatetime)
        self.addCleanup(patcher.stop)
        patcher.start()

    def test_something(self):
        self.assertEqual(datetime.datetime.now(), datetime.datetime(2012, 5, 18))
        self.time = datetime.datetime(2012, 5, 20)
        self.assertEqual(datetime.datetime.now(), datetime.datetime(2012, 5, 20))