def _time_conver_ms_to_timestring(millis):
    """ convert millisecond time delta as a float into a string describing time in HH:mm:ss """
    millis = float(millis)
    seconds = millis / 1000 % 60
    minutes = millis / (1000 * 60) % 60
    hours = millis / (1000 * 60 * 60) % 24
    return '{0:02d}:{1:02d}.{2:02d}'.format(int(hours), int(minutes), int(
        seconds))
