how to extend the dayes of demo version application

This is how you can do. Create a database contain accepted keys. Create a new user which may only select data. When customer pays insert key into the db and tell him the key. Then the program either connects to the db directly, to an server or to an service. Then if the key is right the program gets unlocked.

Another way is to give all applications a uniqe ID which you store somewhere. It would work pretty much the same way but without the need of giving keys :)

I know this is just theory but I don't know anything about your application or your setups so without it I cannot assist you futher ;)