There is no 100% accurate clock, but the day time is 5 hours long and the night is 1.5 hours. During the day I use the sun as my clock. It's usually visible, and when it is directly overhead (at zenith) it is "noon". Will a little practicd you can tell fairly accurately how much daylight is left. As far as I know there are no clocks in the game, although I think it would be a really cool move on devs part to add some sort of sun dials or some alternate means of telling time.