Keeping Time

The importance of keeping a record of game time was something that in my youth was overlooked. In those days, we were happy to stick to the hack'n'slash campaign where no door or monster was safe, and there was always adequate food and light available. Sure there were consequences to actions made in the game world, but now that I think of it they were limited to the perils of unprepared for combat encounters. Never did the PC's have to worry about where the next meal was coming from, or if they delved too deeply into the dank realms below to the point where their torches were sure to go out leaving them to a certain "never to be seen again" death.

Now in my studies of the early editions of our favorite hobby, it is clear that the passage of time adds an element that I never knew I was missing. In fact, thinking about the passage of time opens up a whole new world of adventure ideas such as the obvious running out of food while deep underground adventure. The PC's suddenly find that they need to abandon their original quest for a more pressing one of pure survival. What is safe to eat deep underground anyway? The ideas could continue on and on, but my question is simple. How do you keep track? I did keep track of a calendar, but that is not exactly the same as keeping the hours accounted for. Discuss!

UPDATE: A big thank you to Chgowiz for sending me this link to HEYMIKEY.ORG. Here you will find a great set of time keeping sheets that allow you to check off each round/turn as you go. I plan to put these to use in my new campaign.