Hello, I have a archievements db and user can gain the archievements. Sofar its working great but how to add a timestamp to display when the user has reached that archievement? 
It may be possiable.
I don’t fully follow the question but I think what you want here is to use visibility settings on the achievement image on the screen that is filtered by date/time properties being met correctly.
When you create the achievement (which I asume you are doing by using actions) just add make sure the timestamp is populated with current time.