Auto Change Passwords

[AndyShuter]

I'm just about to sell one of my first applications As the customer is "renting" the sortware by the month, I'd like a way of making the password change every month automatically so he would have to call me for the new password each time.

I suspect the best way to do it would be to use an algorithm that uses the date as it's underlying value - but how do I put it into code?

Any suiggestions would be greatly appreciated