having a lockdown function that does not rely on the computer time

I am writing software for a friend and this will ultimately be something that they distribute to their distributors. Each distributor must pay a fee each month to use it. if they do not pay then the software needs to shut down. Here is how I am thinking this will work, but I am open to ideas!

-Program opens.
-go online and look in a file on server to see if persons name is in file.
--if so then software continues working
--if not then warn user they need to pay or software shuts down in 7 days


Here are problems I am thinking of:

what if they are in a location without internet for more that 7 days
--potential solution: also have a manual way to enter a key to keep it running.

if the software checks the computers date, then how do I know if the user is setting the day back, so as to keep using the software? They may take down their internet connection and just keep the calendar rolled back.

what if they leave the software running without shutting it down
--potential solution: Have the software check online every 24 hours? Possible might get into problems where people are setting calendar back?



what about if they need to enter a key manually? how will I generate a key that will instruct the computer when this temp key is going to expire? one thought is to perform some math on the date of expiration to make it look random. The program would unencrypt the number to obtain the date. Still might have the problem of the calendar being set back.

Is there a better way to do this all together? Am I thinking to hard. Are most people smart enough to know how to get around some of these blocks?

Thanks, Victor.