def get_occurrence_times_for_event(event):
    """
    Return a tuple with two sets containing the (start, end) datetimes of an
    Event's Occurrences, or the original start datetime if an Occurrence's
    start was modified by a user.
    """
    occurrences_starts = set()
    occurrences_ends = set()
    for start, original_start, end, original_end in event.occurrences.all(
        ).values_list('start', 'original_start', 'end', 'original_end'):
        occurrences_starts.add(original_start or start)
        occurrences_ends.add(original_end or end)
    return occurrences_starts, occurrences_ends
