How to set the program to run on limited time or demo?

i'am a student and has a internship with government office for which this project will be tested.
my coach on that office wanted a computerized inventory, that is why he asked me if i know some
programming skills.

he is a demon. shout at me many times -.-

so what i wanted to do is to turn this program into a demo version or something that will not be able
to use if the time is reached or certain date has been reach. in this case i can get revenge on him
bwahahaha xD