I wouldn't take the numbers so seriously.
9/11 happened 10 years ago. In casual conversation, you round numbers. 10, 15, 20.
Yeah, but, generally speaking, nobody rounds 18 down to 15, they round it up to 20. And I would totally agree that there is some wiggle room if not for the fact that both Kirk and Khan independently
give the 15 year figure as the amount of time since thier last meeting.
"Canon is only important to certain people because they have to cling to their knowledge of the minutiae. Open your mind! Be a Star Trek fan and open your mind and say, 'Where does Star Trek want to take me now'." - Leonard Nimoy