Time is a resource. Everyone knows it has to be managed. And on the Discworld that 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 monks of time. Because it will stop time. And that will only be the start of everyone's problems.
The last archive contains a pdf version of the book.