def timer(start, end):
    """ Give duration time between 2 times in hh:mm:ss.

    Parameters
    ----------
    start: float
        the starting time.
    end: float
        the ending time.
    """
    hours, rem = divmod(end - start, 3600)
    minutes, seconds = divmod(rem, 60)
    return '{:0>2}:{:0>2}:{:05.2f}'.format(int(hours), int(minutes), seconds)
