A simple way would be to use the result of
Time.get_unix_time_from_system() as your base time value. That's a value representing "elapsed seconds" since the Unix epoch time (Jan 1, 1970). It's easy to manage as it's just an integer value. So, if you store that value when someone quits your game, and you get a new copy of that value when the game starts, you'd have two points in time, each represented by an integer value.
From there, simply subtracting the first value from the second results in the number of real-time seconds that have elapsed between those two time stamps. Using that value, you can do whatever is appropriate in your game.