If it is for Licensing and that is the avenue you want to go down then as James suggested that is the best way to go about it.

Assume you start from setup, then you should automatically save a pre-dated file somewhere with the application files, as this will act as an initialisation that you can compare against and prevent any system date tampering before the application is run.

Ensure you don't save the file somewhere obvious, or if you must then run the data through a cypher and ensure the file is needed to start the app along with a certain key hidden within the data.

You don't have to save the file as a text file either, infact, hide it further by assigning a dummy extension... remeber unless your app is for people with an intimate knowledge of computers (Insert your own 3inch floppy gag) then they will be fairly ignorant of the watsit thingymajigs. When the app is started firstly check the system date, if it is earlier than that already on the file delete the key (License) if it isn't then update the hidden date file.

That should sort it for you, simple, though not hack proof by any means.