The trick here is to display game time as against real time. Having a timer start when the game starts and display hour:min:sec is no big deal, but game time is something else entirely.
Game time is different on different game speeds, and the real spanner in the works is lag, which can mess with the game time/real time equation in many different ways. To display game time correctly you would need to either access an internal game timer or hook a couner into one of the main loops in the game engine.