It's exactly like learning another language - you will become a programmer (at least you will if you stick with it). Don't expect to be able to program with copy and paste, the examples you find on the web are just to aid understanding and help you write your own.
Copy link to clipboard
If you post your script and the field names we might be able to help you adjust it.
If you want someone to do it for you, you'll probably need to pay them.
If you wish to learn how to do it yourself, these are very good tutorials about this subject:
If you're interested, by the way, I've developed a (paid-for) tool that allows you to set up such calculations quite easily and without having to write any code yourself. You can find it here: Custom-made Adobe Scripts: Acrobat -- Apply Automatic Date Calculation