Start and end times can be provided as optional arguments.
But note that because of YouTube limitations, these times apply only in a few situations as follows:
Start and end are honored once when playing the embed with its big red button. If you replay the embed, it plays the whole video, not just the part between start and end.
Start is passed to YouTube only through the "Watch on YouTube" link that appears at lower left before watching the embed. If you play the embed and then click the YouTube link at lower right, then YouTube will continue playing the video starting at its current position in the embed, not at the specified start time.
End is never passed to YouTube from the embed, so clicking any button to watch on YouTube will play the full length of the video, not stopping at the specified end time.
Here is an example. I suggest to start by pressing the big red play button and observing that the clip ends automatically after 12 seconds have passed. Then you can refresh this page to check the other behaviors described above.