def get_ending_datetime_of_timetable(timetable):
    """
    Get the ending_datetime of a timetable, which corresponds to
    the arrival_datetime of the last timetable entry.

    :param timetable: timetable_document
    :return: None (Updates timetable)
    """
    timetable_entries = timetable.get('timetable_entries')
    ending_timetable_entry = timetable_entries[len(timetable_entries) - 1]
    ending_datetime_of_timetable = ending_timetable_entry.get('arrival_datetime')
    return ending_datetime_of_timetable