def get_end_time(starttime, num, span):
    """ Utility function
    """
    if not starttime or len(starttime) < 4:
        return ''
    m = int(starttime[2:])
    hour = int(starttime[:2]) + (m + int(num) * span) / 60
    minute = (m + int(num) * span) % 60
    return str(hour).zfill(2) + str(minute).zfill(2)
