You can learn how to add custom start and end time to your videos in this guide - How to Use Custom Start/End Time
This video starts at 1:00 and ends at 1:30, making it 30 seconds long:
This shows use of a custom start/end time in a playlist of MP4 videos. The first video plays in the middle for 5 seconds, the second video for 2 seconds only.
The same can be done with a playlist of YouTube videos. First video plays for 3 seconds, the second video for 7.