Time is a source. Everyone knows it must be maintained. You mess with it at the peril. You can allow it move fast or gradually but everything you mustn’t do is allow it to stop.

Over the Discworld period management is the job from the Monks of Background, who shop it and pump it in the places where it’s wasted (like the underwater – just how much period will a codfish need?) to places like metropolitan areas, where there’s by no means enough time.

But the building of the world’s 1st truly accurate clock begins a race against, well, period for Lu Tze and his apprentice Lobsang Ludd. Because it will stop period. And when time stands still, everything halts with it. Then, there really is no future.