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. 
 
The Monks of History have the glamorous job of time management in the Discworld. They 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 with the construction of the world's first truly accurate clock starts a race against, well, time begins for History monk for Lu Tze and his suspiciously talented apprentice Lobsang Ludd. 
 
Because a truly accurate clock will stop time.
 
And when time stands still, everything in human existence stops with it. Then, there really is no future.