The Doomsday Clock as a tangible narrative element

By default, while the players are aware that their time is limited, their heroes do not. There are a few reasons for this; the most important ones being (1) sustaining some sense of mystery to unfold, and (2) encouraging a wider variety of story premises to occur—from the perspective that it might be difficult, for example, for a Guide to continually pitch and run games where the conceit is “everybody knows their Doom is coming”.

That said, there is literally nothing stopping you from doing exactly that.

It doesn’t have to be “meta”—the awareness of impending doom can be woven into the narrative. For example, a prophecy, a rumor of invasion in X days, or even a cryptic grandmother who tells the heroes outright that their time is limited. For more subtler cues, you can embed it into a repeating motif (“a clock perpetually on the shadows, ticking with an insistent hollow sound”) or tie it to a hero’s ability or inventory item. In one game I ran, one of the heroes had a parrot that could pick cards from a deck to give hints about the immediate future; by the end of the game, the cards it drew grew more and more dire.