How to put the validation expression on time

Question

Dear,

How to put the validation expression on time.

I have to column Start & End both are datetime.

In time is 24hr format.

What i want, i want to allow user only enter 15 min difference

Example data look like this

Start          End

0:00          0:15 ----------> here cannot enter 0:16 or 0:17 or 0:18 so on

0:15          0:30

0:30          0:45

How to do the validation on client side

Thanks

Basit.