Calculate the difference between to date

An aside: I would personally strongly recommend against using VBScript for new code.  Microsoft has already announced that VBScript is on a (slow) deprecation path; it makes sense to use alternative options for new functionality, rather than writing something that is guaranteed to need to be re-created in a few years.

Seems like something you could use the property calculator for.
Have you tried it?

While I agree with Craigs comment there may be situations where a quick short term solution i required. You can get it done with the DateAdd function as described here: https://www.w3schools.com/asp/func_dateadd.asp

But before you go down that road please look at other solutions like the PropertyCalculator or Extension Kit.

Thank you for the answers

Property calc has mode Period Lenght, this should work for you.