def with_start(self, t, change_end=True):
        """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'.

        These changes are also applied to the ``audio`` and ``mask``
        clips of the current clip, if they exist.

        note::
          The start and end attribute of a clip define when a clip will start
          playing when used in a composite video clip, not the start time of
          the clip itself.

          i.e: with_start(10) mean the clip will still start at his first frame,
          but if used in a composite video clip it will only start to show at
          10 seconds.

        Parameters
        ----------

        t : float or tuple or str
          New ``start`` attribute value for the clip.

        change_end : bool optional
          Indicates if the ``end`` attribute value must be changed accordingly,
          if possible. If ``change_end=True`` and the clip has a ``duration``
          attribute, the ``end`` attribute 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``.
        """
        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
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'.

        These changes are also applied to the ``audio`` and ``mask``
        clips of the current clip, if they exist.

        note::
          The start and end attribute of a clip define when a clip will start
          playing when used in a composite video clip, not the start time of
          the clip itself.

          i.e: with_start(10) mean the clip will still start at his first frame,
          but if used in a composite video clip it will only start to show at
          10 seconds.

        Parameters
        ----------

        t : float or tuple or str
          New ``start`` attribute value for the clip.

        change_end : bool optional
          Indicates if the ``end`` attribute value must be changed accordingly,
          if possible. If ``change_end=True`` and the clip has a ``duration``
          attribute, the ``end`` attribute 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``.
        
moviepy/Clip.py