What class are you using to represent the Date? Is it this? You will have to go through its documentation and work out which methods add 15 minutes, and which methods return the milliseconds since 1st Jan 1970 as a long. Then you can compare the value with sysdate, which you can obtain from methods of the System class.
The rest is easy.
Would anybody like some fudge? I made it an hour ago. And it goes well with a tiny ad ...
a bit of art, as a gift, that will fit in a stocking