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.
I’ve looked at a lot of different solutions, and in my humble opinion Aspose is the way to go. Here’s the link: http://aspose.com