I can't take credit for the idea, I saw it on YouTube some time ago. I think it was Jim's fix-it shop or something like that.
Anyway, I finally set mine up. I know some will find it silly or not at all useful but I think it's neat. It's an analog clock that is attached to my foot switch. When the saw is running the clock is running. I can simply set it to 12 and count how long I've worked on a project. I can take a break, eat a sandwich, take a nap and when I get back and start again the timer goes again.
I suppose you could use this to price your work, but for me it's more curiosity. I just want to see how long certain projects take. I found the clock on ebay for $10 so to me it was worth it. I will say that there weren't many analog clocks on ebay when I got this one though so it could be hard to source. It has to be analog because digital would reset every time you killed power to it.