Minecraft wiki entry for Daylight_detector, associated text: Because you can measure exactly what light level it is, you can make a clock. For example, for every light level it is, 1 light goes on a board. When it is night, the word "Night" is illuminated on the board displaying light level or time. This can be useful for servers, especially if you do not have the ability to create one using piston memory, or do not wish for one as it can be complicated or laggy.