CLOCK

A compass points at a fixed spot, but a clock is the tool for the question that changes every day: how much of it is left before something underground starts hunting again. The Librarian sells one for 5 emeralds once he has reached expert.

It sits on his shelf next to the other utility items rather than the books, since it is a tool rather than reading material. A crafting grid makes the same item from gold ingots and redstone, so the trade is really a question of whether four gold ingots and a redstone dust are easier found than 5 spare emeralds.

One seller, one price: the Librarian at expert asks 5 emeralds for 1 clock, a figure written directly into the trade file rather than rolled, so a second Librarian at the same tier quotes exactly the same.

Nothing buys a clock back off you, and the offer runs 12 times before it locks and the villager needs to walk home to his lectern to restock.

Clock sells for as little as 5 emeralds, from the Librarian at the Expert tier, and 5 of the other 6 cost less. The price is fixed rather than rolled, so a second Librarian will quote you exactly the same.

The Librarian at its expert tier asks 5 emeralds for a clock. 8 of the 72 villagers that pay emeralds, emptied out:

5 diamonds against 120 flint for the same 5 emeralds, 24 times the carrying, and the shortest wait is the Shepherd at 3.1 real minutes of shelf, gathering not counted.

Every clock offer locks after 12 uses, and with two restocks allowed per Minecraft day, one Librarian is good for 24 of these trades a day and no more.

It sits at expert, level 4 of five, which a villager reaches at 150 trade experience, so a freshly employed one has real climbing to do before a clock shows up on his shelf.

Trading with him at earlier tiers pays toward that climb, this offer worth 15 experience a use, meaning 7 of them alone would clear the next threshold if he already had it unlocked.

12 clocks empty one slot, as many clicks and no more. The restock is where the real time sits: the game holds the offer for 2 real minutes before the villager can refill it, and a Minecraft day of 20 real minutes only fits two of those windows.

One Librarian therefore hands over 24 clocks for 120 emeralds across a 20-minute day, the most one trader will ever produce before the day runs out from under him.

Clearing the shelf costs 60 emeralds: 12 clocks at 5 emeralds each before the Librarian needs to restock.

Where the emeralds come from: the same Librarian pays an emerald per 2 book and quills, a supply worth building ahead of a shelf-clearing run.

60 emeralds for a full slot clears the bar where curing a zombie Librarian first is worth the golden apple, since the discount it locks in keeps paying out on every clock after. How curing works.

Also on the Librarian's list: compass, yellow candle, red candle, paper, bookshelf, and enchanted book.

The dial follows the sun and moon, which is why it reads nothing at all in the Nether and the End: those dimensions have no day. Underground in the overworld it keeps working, and that is the point of carrying one, since a mine gives you no other way to know whether it is safe above.

Four gold ingots around a redstone dust makes one, so the cost is gold rather than effort. In a frame it keeps ticking on the wall, which is the tidiest way to give a base a working clock without holding it.

A clock is four gold ingots around one redstone dust, which is not cheap for something that only tells you the time. That price is why most people meet their first one as loot or as a trade rather than making it.

In an item frame it keeps working, so a clock on a wall underground is the standard way a base knows whether it is safe to go outside without cutting a window.

The item also stacks, which the compass does too, and both of them are the sort of thing worth keeping one of in the shulker box rather than in a pocket.