'Returns a copy of the clip, with the ``start`` attribute set
to ``t``, which can be expressed in seconds (15.35), in (min, sec),
in (hour, min, sec), or as a string: \'01:03:05.35\'.
If ``change_end=True`` and the clip has a ``duration`` attribute,
the ``end`` atrribute of the clip will be updated to
``start+duration``.
If ``change_end=False`` and the clip has a ``end`` attribute,
the ``duration`` attribute of the clip will be updated to
``end-start``
These changes are also applied to the ``audio`` and ``mask``
clips of the current clip, if they exist.'

@apply_to_mask
@apply_to_audio
@convert_to_seconds(['t'])
@outplace
def set_start(self, t, change_end=True):

    self.start = t
    if ((self.duration is not None) and change_end):
        self.end = (t + self.duration)
    elif (self.end is not None):
        self.duration = (self.end - self.start)
