No need - Contro’s goal window hack can display a timer. You just need some ram to store it in. Iirc his displays game time by default.
https://github.com/masterofcontroversy/ModularGoalWindow
My edited version: https://github.com/Veslyquix/Roguelike/blob/main/Patches/ModularGoalWindow/src/DrawTimeText.c#L40
You can see a 10 second per turn countdown used by me here: Make A Really Terrific Hack: MARTH - #10 by Vesly
@Contro official release when? Add a countdown version, too 
Limited time fe aside, this is some neat stuff, 7743. It made me start thinking about asm puzzles to solve ingame to progress somehow. Not sure how that would work.