Perhaps you did not uderstand me.
It's just dummy code that gives an idea of what you could do, returning either 1 or 0 is just a boolean expression or calling it as a sub is just to help me explain what i ment to do, this is not a final solution, just an idea on how you could do something like this. How you implement it in your code is up to the programmer.
The thing i ment to do, is showing a way to use dates to see if you've got a valid license, not the implementation itself.
Anyway, using a license code is not an ideal idea anyway (as i said before), it allways can be hacked out. The OP however did ask not to be 100% safe on hidding source, just make it more difficult.
The idea of calling for certin functions to your own server is a bad idea as well, it's not failsafe. It however gives fresh ideas to the OP so he can think of a proper solution to his problem.
"We all agree on the necessity of compromise. We just can't agree on when it's necessary to compromise." - Larry Wall.