New(newest) post I need a New Flag Darn it

Alright so heres what Im dealing with. I want to display to a user that a post is new to them. Well, I know I can do this with basic css and just put a new tag on the background of an and that works perfectly but heres the problem, a user can clear oute the cache and boom now ALLLLLL posts on the site will have a new tag. This is an issue.

I was wondering if maybe there is a way to display a new flag based upon a particular amount of time lets say, like for 48 hrs after post it will display a NEW flag. I cant figure out a way nor have I been able to search on here exactly what I’m trying to do