Unless you have a total lockdown on the db, including its tables, this is almost impossible, but I guess that depends on the sophistication of your target user. It's likely that I could expose which field I need to update, and then update the date you're using to calculate the 15 days. Or I could probably get by with back dating my Windows system date to get around your barrier.
If you want to block most hacks, you'd probably have to write code to add a registry key and a key value. Even then, that will likely fail in a corporate setting where a user can use the db but isn't allowed to modify the registry. In that case, you'd need to use an installer that their IT department would trust.
If you want to try something else, you could create and set a custom db property and distribute the db with it having some value such as zero. Then when it first runs, make the value a date value of Date + 15 and compare the property value each time it runs. On first run, the test would have to be IF property = 0 Then - not IF Date>property value because on first run, any date will be greater than 0.
I confess I've never tried the latter but have used the registry method.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.