> 'seconds' - %H:%M:%S
> 'us' - %H:%M:%S.%f

'us' is not consistent with the datetime module: it should be 'microseconds.

>>> datetime.datetime.now().second
50
>>> datetime.timedelta(seconds=1)
datetime.timedelta(0, 1)

>>> datetime.datetime.now().microsecond
123710
>>> datetime.timedelta(microseconds=1)
datetime.timedelta(0, 0, 1)