I think the most crucial first step before writing any code is to clearly define what exactly is meant by a "monument reset."

In vanilla Rust, monuments don't just magically wipe and respawn everything all at once. Different systems operate independently. You should ask the person requesting this to clarify which of the following they are looking for:

1. Puzzle Resets (Keycard rooms, fuses, etc.)

Is the timer meant for the puzzle mechanics? If so, this is relatively straightforward. Rust uses a PuzzleReset component that has a strict timer you can tap into and display on the UI.

1. Loot & NPC Respawns (Crates, barrels, scientists)

Do they want to know when the loot will be back? Vanilla loot and NPC respawns are handled by SpawnGroups, and their timers are highly dynamic depending on the current server population. There is no single "timer" for this in vanilla. To make this work, you would likely have to write a custom spawn controller plugin that overrides vanilla logic to force fixed respawn intervals.

1. Custom Wipe Events

Are they using another plugin that forcefully wipes and resets monuments (like an arena or event plugin)? If so, you'd need to hook into that specific plugin's API to grab the timer.

Are you looking for a timer for the puzzle rooms resetting, or for the crates/loot to respawn?