Dungeon clocks are a way to incentivize longer adventuring days by giving more weight to rests and time management. As the players interact with the dungeon and its denizens, new creatures awaken, factions progress, alliances shift, and outside forces interfere.
The example dungeon used here is for ~4 level 3 characters using Dungeons and Dragons 5th Edition (all creatures found in the Monster Manual), but the underlying mechanic can be used at any level in any system. It is specifically for dungeons though, so using it for e.g. overworld encounters would require some tweaking.
The instructions are on the one-page design, and all stats necessary to play should be on the dungeon sheet besides it. There are no traps or locked doors unless specified so ignore the various door graphics.
Step 4’s “tell the players” is vital – if the players don’t understand how this mechanic works, they don’t have the information to make meaningful decisions about when to rest. I’d rather tell the players about this and not actually use it, than use it without telling them, that’s how important that part is.
Hearty bonuses are referenced once or twice, and while originally cut, I did later come back and develop that into a complementary houserule: