Warnings for Calendar views with end date unset

When using the Calendar view templates on a field set to capture start and end dates, but the use of an end date is *not required*, uninitialised date warnings appear:

Warning: DateTime::getTimezone() [datetime.gettimezone]: The DateTime object has not been correctly initialized by its constructor in DateObject->__construct()
Warning: DateTime::format() [datetime.format]: The DateTime object has not been correctly initialized by its constructor in DateObject->format()

This appears to be caused by the "value2" key being set, but NULL.