At my last full-time job as an editor, I used the task timer in Microsoft Outlook (not sure which version it was, though).
Once a client is listed in your contacts, you can set up a task, call it “Production 1.” There’s a timer you can start and stop as long as you keep that task window open. You can Alt-Tab over to it when you have to take a phone call from another client, etc.
The timer keeps track of your time in user-definable increments — we used tenths of an hour. That way, when the DP/Business Manager called down the hallway “How long did you work on the ____ project?” I could quickly and easily look it up. (Later, I got into the habit of emailing him the total after we sent out dubs of the commercial, thus saving the down-the-hallway yelling.)
Hope this helps as well.
Jay