Cool! I used to have my own back in 2005/2006 I remember. Mine was written in AHK and worked by periodically asking me what I was doing.

Since then I switched to Manic Time which just logs the open application and window title. (In the free version, more features in the paid version which I use now even if the free version is really good.)

The benefit of your approach and my AHK script is it encourages thinking about how one uses the time during the day and not only towards the end of the week when the timesheets are due.