the easiest way is to have a mde, not an mdb
then put code in the startup to check a date, and close if you have reached the date
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having a period for a demo is harder
since there HAS to be something external to store the expiry information
this could be a file, a registry entry etc etc
you could encrypt the information in this, but it does potentially give a way for a user to identify the mechanism, and circumvent it.
instead of a date, you could count a number of uses, but there remains the potential for a user to establish how/where you are controlling this variable
having said all this, if you use an expiry date, the users may be able to just change the computers system date to get round this.