This one might not make a lot of sense at first.
For a school assignment we had to think of a new type of clock. I choose to use my skin; since I have psoriasis, my skin renewes itself too fast, resulting in red 'spots' on my body. This decreases during the summer and increases during the winter. Also, it increases when I have stress. This happens mainly during the evaluation periods at school (4 times a year). So you can (kinda) see what period it is by looking at the amount of red on my skin.