How to make an out-of-game timer in Unity game?

Question

C++ / C#

Greetings, I'm making a simple mobile phone on unity. I came up with the idea to make a gift that can be opened every 3 hours. So here's how to implement a countdown so that it runs when the game is not running. Prompt the documentation or article where it was described how it can be implemented.

Answer 1

1. Remember the time to collect the reward
2. Watch the time now
3. Compare 2 times