The Ticking Clock: Time-based RPG objectives

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I've started planning my next adventure module and I'm thinking of including a ticking clock scenario. Essentially, the players will have a limited amount of in-game time to complete an objective. If they take too much time or ignore the task, there will be an irrevocable change to the game setting. However, unlike video games, there's no hard fail state in tabletop RPGs, short of a total party kill. If the characters fail the task, the world keeps going.

I've discovered there's a fine balance in planning ticking clock scenarios. Make it too difficult and the players won't engage. Too easy and the players get bored. I also don't want to over-complicate things for the dungeon master or players by having loads of calculations to track. I'm happy to do the heavy lifting when it comes to calculations but the DM should be able to pick it up and say "in the amount of time you have, you can do this or this but not both."

I'm also thinking about when to trigger the ticking clock. Do I let the players get a feel for the new area first or drop them in it right away? Does it happen before or after the dungeon? At the moment I have it in my head that they'll need to get through the dungeon before the deadline but this limits the amount of exploration and interaction.

As with my previous adventure, I'll playtest the new module to get a better feel of what works best, both for the DM and the players.

Do you have any experience designing or running an RPG ticking clock? Any pitfalls I should look out for?