Go to the 15 sec mark in that mov and hit “Alt [“, this will trim the inpoint to the 15 sec mark of the movie. Then slide it Around until it starts in the composition where you want it to.
You can also double click the mov layer and set the inpoint in that layer window, but I just use, Alt [, to trim. By the way, Alt ], trim putting the outpoint in that spot.
If you meant to not start the mov until 15 sec into your composition then go to the 15 sec spot in the timeline and then import drag in your mov from the project window.