How can I track how much time has passed since a game was turned off?

I’m making a countdown timer that needs to work when the player turns the game off. When the player turns the game back on, it should still be able to countdown any remaining time left or let the player know if the timer has finished counting down. However, I’m not sure what to use to determine how much time has passed. Since the countdown timer needs to be able to work even after the game is turned off, I need to be able to store when the timer started. I was thinking that I could use a DateTime object to store when the timer started, that way I have something that knows exactly when the timer began when comparing how much time has passed. Would this be possible, or is there something else that would work best?

You can use the PlayerPrefs class to save data for later use.
That data you can recall on starting the app to get back the old timer state.

Adding on to Malzbier’s answer, you could store the timer’s value somewhere (either in PlayerPrefs or some other format) and also store the system’s time. Then at start up get the system’s time and compare the two to figure out how much time passed on the stored timer. I haven’t really used the system time at all but here’s a few links I found which might help

https://forum.unity3d.com/threads/how-to-get-system-time-in-unity.29667/
http://docs.go-mono.com/index.aspx?link=T:System.DateTime