def timer(start, end):
    """
    This function calcualtes the elapsed time between the start of the function until the end of this.
    :param: float: starting time.
    :param: float: ending time.
    :return: str: time elapsed.
    """
    hours, rem = divmod(end - start, 3600)
    minutes, seconds = divmod(rem, 60)
    return '{:0>2}:{:0>2}:{:05.2f}'.format(int(hours), int(minutes), seconds)
