Hi Saeed,
I will give you my thoughts here since it might also interest others.
I understand you have a startup script and a record in a table that the user can not access, where you save the allowed date interval. It also sounds as if the interval is set on the first opening of the database.
If this is so, we have a setup record that is hidden from the user, and we can now use it in different ways:
Demo
Fifteen days for your solution to run sounds short, so is it a demo? If so, maybe change from a date interval to time in hours. Instead of 15 days, give them 50 hours to check out your solution.
Add a start time field in the record.
Add a countdown field in the record.
Add a script that will run when the solution is closed.
Then, when the startup script runs, set the start time, and in the shutdown script, set the countdown field with time left. Using the countdown field, you can easily show it at startup to inform the user how much time they have left.
Date 1
I remember back in the last century when I set back the clock to cheat an end date, but that is not working anymore since that would impact so many other computer functions since everything is so complex and interconnected. (I am talking about a time before the internet compared to today).
Anyhow, a simple fix is the following:
Add a last date field in the record.
Add a script that will run when the solution is closed.
Then, when the shutdown script runs, set the current date, and the next time the startup script runs, check that the current date is ≥ the last date.
You can, of course, set the current date in the startup script, but if the user leaves your solution open over the night, it might be one day behind.
Date 2
Since everyone is connected to the internet at all times nowadays and there are a lot of things that won't work correctly on the computer if not, you can also use that to your advantage:
In the startup script, send a request to a "Current time API", and you will always have the correct time even if the clock in the computer is broken. I use the following free API that returns a JSON or .txt response that is easy to parse.
WorldTimeAPI