If you are looking for REAL clock, go here.
Exactly, it's not a clock at all. It just tells you if it's night or day outside. This is the most simple thing that you can make. It's useful in mines, maybe sometimes in deep caves.
It can be improved with S/R latches and signs, but that can destroy the main feature of this circuit - simplicity. If tou want to save some materials, you can remove the NOT Gate, but in the night it won't light up the cave (it will make light in day, and that's useless).
Just if you don't know - the end marked "A" should be in the cave or mine, the other end (marked "B") should be outside (best place is somewhere on the hill or roof). Due to easy construction it's not necessary to put a world file here.