This blog post suggests that John Carmack uses a timer to measure his productivity - http://bookofhook.blogspot.in/2013/03/smart-guy-productivity-pitfalls.html.
I wonder if a substantial number of people do these kinds of hack. Do any of you use some kind of timer or application to keep track of your productive time? Do you maintain logs of what you got done today? And if you do, how much do you think it helps you.