How do I add items over time?

I am trying to figure out how to reload/recharge an item in the players inventory over time.
For example, let’s say I want to add one health potion to the players inventory
once every six hours, regardless of if the game is even turned on. I can do this easily enough while the game is running, but what I can’t figure out is how to basically reference a system clock or whatever it may be, so if there is one health potion in your inventory and you turn the game off and come back 12 hours later there would be three.
Any suggestions would be great. Thanks.

offline game?

just save the time when the player quits.

System.DateTime.Now

when he starts the game again … yeah the rest shouldn’t be the problem.

You could store the time for the next item in the PlayerPrefs, this data is persistent between games.