Fixes bug 1069039 - Adjust start times for event assignments

Start times for Event Assignment iCal feeds adjusted to 30
minutes prior to event start time in order to allow staff
sufficient set-up time for events.
You should add a doc string here. Something like
```python
def test_calendar_dtstart(self):
    """test the behavior of the `DTSTART` value in the iCal feed. ..."""
```