I distinctly remember having seen a parameter under « youtube.com/watch/ » which indicated when a video was to stop playing, much as the "t" parameter indicated the starting point. It had a multi-letter name of some sort and took a value in seconds just as the "t" parameter did. I have a few videos which will only play under « youtube.com/watch/ » and it strikes me as unfair that these videos can't be played with an ending point as well. Whatever information you can provide will be deeply appreciated.
The parameter is callend 'end'.
This parameter specifies the time, measured in seconds from the start of the video, when the player should stop playing the video. The parameter value is a positive integer.
Note that the time is measured from the beginning of the video and not from either the value of the start player parameter or the startSeconds parameter, which is used in YouTube Player API functions for loading or queueing a video.
Look here for all available parameters