You could create a table with 2 fields, start date, and days left. The start date would be a random date, let's say 01/01/1980, and the days left would be the time period for your demo. Each time the application starts, simply check if date() # start date then simply subtract 1 from the days left field. When days left = 0, prevent the application from being run.

The only way the user could break this code would be to set the date on his computer to 01/01/1980 each time the system started. As the user has no way of knowing this start date, it's very unlikely they'll work it out. If you set this start date to something really obscure, perhaps 30/02/1999 which is an impossible date, then the user could never set their date to this, even if they knew what this start date was.

Derek