I just look at my commit times But my workflow is kinda weird, i change code and I commit which then pushes it to my development server. I do it this way because I always use different computers throughout the day/week. So that way I always have the latest version of my development code. Then when I finnish a feature I push it from my dev repository to the test repository where I use git like it should be used. But because I have this development git repository it is easy to see my work times. I still want to automate it though, now i'm just starting up qgit and do the manual lookup for a week and bill that to my clients. I do all my projects this way.

Also recently started using this: https://github.com/gurgeh/selfspy It gives quite awesome data, and most of my workflow is known, so maybe I could hack something together that logs things it considers work (but since I change pc so often I don't know if that will really work). Should have a procrastination/work/documentation list for your browser though to make it more effective.