Just record your starting time and then calculate the value whenever you need it. It should just be the elapsed game time (in days) multiplied by 288 (24 hours and another 12 to get the 5-minute interval).
((Utility.GetCurrentGameTime() - InitialGameTime) * 288) as int
You can either recalculate in the event where you need the value, or if you're using it for some condition, you'll probably want to have it updated regularly with something more like this.
float InitialGameTimeint FiveMinuteElapsedCountFunction GettingStarted() InitialGameTime = Utility.GetCurrentGameTime() RegisterForSingleUpdateGameTime(0.0833) ; approx. 5 minEndFunctionEvent OnUpdateGameTime() FiveMinuteElapsedCount = ((Utility.GetCurrentGameTime() - InitialGameTime) * 288) as int RegisterForSingleUpdateGameTime(0.0833) ; approx. 5 minEndEvent