Maybe you could save the time when player quits and then when game is restarted compare the time.
What if the game crashes or the player presses the home button?
Rather you could save the time when a certain event started, for instance when the player started building a building and in case of a crash or the game being closed on start you could compare the current time and the time the event started.
This obviously could be manipulated by the player by changing the devices time and date settings.