The problem is that you've named the relationship "Calendar" in your custom datetime class, which is something I didn't expect, so I just have to search for every reference to CalendarID and make sure it's explicitly named CalendarDateTime.CalendarID.
I can't see any reason why this would still be happening if you're on the latest SVN. Ensure that line 201 of CalendarDateTime.php reads:
"is_announcement = 1 AND CalendarDateTime.CalendarID = {$calendarID}"
and not
CalendarID = {$calendarID}