def format_time(start, end):
    """
    Format length of time between start and end.

    :param start: the start time
    :param end: the end time
    :return: a formatted string of hours, minutes, and seconds
    """
    hours, rem = divmod(end-start, 3600)
    minutes, seconds = divmod(rem, 60)
    return "{:0>2}:{:0>2}:{:05.2f}".format(int(hours), int(minutes), seconds)