I'm same Seerah .. sounds like you are aiming to stop the addon from working for some reason .. seeing as some addons can work several expansions after they are made ( if they are lucky ) there would not necessarily be a reason to stop the addon from working. You shouldn't need to stop the addon working as the game will tell them if there is a problem and stop it itself, giving the user the opportunity to remove the addon themselves. Any other reason to stop it from working sounds like its against the terms of use.
That said ..
As far as I am aware the OSDate you were calculating is the date on the server the game is running on, not the person computer.
As to anything else you might want to do .. you might want to look at
https://wowpedia.fandom.com/wiki/World_of_Warcraft_API as that should give you the information you need. The Addons section should be able to help you.