how to get difference between two times using javascript

This is not how CP usually works. Most important goal here is to learn and help learning.
You are supposed to try it on your own, and come here when you got stuck with something, with a concrete question about your code, design, etc.
Please have a look to What have you tried?[^] to see a good explanation about what I mean.
Don't forget people here don't get payed. And besides, if we give you a ready-to-go solution, it is not going to help you because you are not going to learn anything from it.