coding expiry date function into existing small program

I have a small program that scrapes data from Ebay website and, if user inputed variable criteria are met, it generates warning notices. I would like to add an "internal expiry date/time feature" that only I could access and then input an expiry Date/Time before sending the program to others. One day before expiry the user would see a message telling them about the expiry and give them contact details to get a new date. I would then like the simplest solution, you could think of, that would make it possible to update that expiry date on their machine. Perhaps I could just send new program or file with new date/time that overwrites old file(s). Obviously if there were any level of security you could add to this simplistic somewhat un-secure solution I have thought of, your ideas are certainly welcome.