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Time is a resource. Everyone knows it has to be managed. You mess with it at your peril. You can let it move fast or slowly but what you mustn't do is allow it to stop.
On the Discworld time management is the job of the Monks of History, who store it and pump it from the places where it's wasted (like the underwater - how much time does a codfish need?) to places like cities, where there's never enough time.
But the construction of the world's first truly accurate clock starts a race against, well, time for Lu Tze and his apprentice Lobsang Ludd. Because it will stop time. And when time stands still, everything stops with it. Then, there really is no future.