I like Google for openness. Good approach I liked is giving users ability to extend some applications with scripting language Google Apps Script.
My experience with GAS was very positive implementing Time Tracking system based on Google services Spreadsheets and Calendar.
Documentation for GAS here: https://developers.google.com/apps-script/
Use your imagination =)
First, we need some storage for work day time bounds. Calendar events is useful here. Next, we need useful interface. Spreadsheets allows you not only watch, but adjust and planning work days to fit 40 hours/week or something in your contract.
Look at this: source code here
How to use
Installation guide you can find in README file, right here
When you arrived at work, open spreadsheet and choose menu TimeTracker->Arrived at work... and enter time when you arrived. Also you can write out comment for this day. Click OK.
When you go home, choose menu TimeTracker->Still on work, it will expand today work to current time.
Feel free to report about bugs, fork and send pull requests