Fix compatibility with HHVM

Some workaround needed to get all the test cases running on HHVM, mainly incorrect behavior of `DateTime->modify()` (see https://github.com/facebook/hhvm/issues/1469)

Before: `FAILURES! Tests: 670, Assertions: 1042, Failures: 2, Errors: 3.`
After: `OK (670 tests, 1045 assertions)`

tested with hhvm master as of 2013-11-31