Jennifer at Clicktime.com has written in detail about making the best use of Google Calandar for tracking your working time. In summary, those seven tips are:
1. Create Clear and Consistent Meeting Names
2. Make Sure Your Calendar Is Up to Date
3. Write Meeting Descriptions
4. All-Day Events
5. Minutes vs. Hours
6. Track When a Meeting Starts, Not When It's Created
7. Don't Forget about Vacation
Read the Google Calendar guide in detail here.