the sun is proof of CLOCKING the day, so to speak, but there is no accurate to the minute/second without an electric microscope. also, the amount of light on a clear day can help with time, once you get used to not locking at clocks and using time, you realize that you can clock your day pretty mutch to the exact time everyday, i could guess when 4:20 is during the day on a clear day, looking at the sun would help a bit. but really its that sence of getting used to the time of the day in your head, but it remains nameless...
P.S. every day i get up and go to school, and i can sense when im late, etc. it is weired, i can wake up and go to school and be there right on time no problem, even if i havent looked at a clock or known the time in weeks.