I notice that I can have a timed dungeon (as per the labyrinth example) and a scroll which says how long there is left to complete the game.
What I would like to do more (it seems much more useful) is have the timer inactive until a certain point. For example, the player can take months to get through my dungeon to level 15 billion

and then come across a reactor which he has to blow up. The game should then say "Right, you have an hour to escape from the dungeon before the reactor blows" and the levels suddenly fill with monsters trying to stop you escaping as a final challenge before the end of the game.
Can I do this, or does the timer have to start at the beginning?