'Get the delta between start and end time.
:param start_time: the start time
:param end_time: the end time
:returns: string -- delta in string H:MM:SS'

@staticmethod
def get_time_delta(start_time, end_time):

    delta = (end_time - start_time)
    return six.text_type(datetime.timedelta(seconds=int(delta)))
