Hey @NotPasi_TV!
You should be able to save the old time, reduce the time by subtracting the new time, and run a “Set Timer by Event” somewhere for this!
Then you’d just pass in the new “Remaining Time” when the timer is run (on game restart), then when the timer is elapsed use a CUSTOM EVENT to reset the “bChestAvailable” boolean! 
Hope that helps! Anything more and we’re gonna have to see some code, though!