User needs a license key to continue using the Macros

I'm looing for ideas on how to have a workbook and all the macros expire after a certain date. I know the code for simply having it expire after a certain date and the work - around is changing your computer time and date to continue using the macro filled workbook. I'm looking for the same idea but user needs to enter a license key to continue using the macros ,after a certain date, in the workbook. I've seen it done before and searched extensivly but nothing even close was found.Any Ideas???
Thanks

It's possible to create a macro that runs everytime that the workbook opens. I would have a hidden worksheet to store the dates everytime someone opens it. Then when the user opens it the macro would compair dates to make sure the user didn't roll their computer date back...
You can protect the macro, workbook, and the spreadsheets, but the passwords in Office are extremely week, and very easy to crack.