Need instructions. Put up christmas lights and the person I bought the house from has a timer hard

Question

Put up christmas lights and the person I bought the house from has a timer hard wired in the garage for the outside outlet it says night sentry on the front and appees to be in good working order but don't know how to use it....It looks pretty old has a on off switch on top and a timer that reads 1-12 but how does it know what time it is?