I think a good idea would be to use a calendar called Timesheet to keep trace of interventions. A zimlet would be needed (and also some database stuffs) to add some informations like client name, project name, task name and if it's billable or not. After that an other Zimlet will be needed to treat the informations and some reporting stuff (billable hours worked for a client by projects and task per month for example, ...).
I think this system will be more modulable and will be more integrated into zimbra (for example all informations can be stored into the main box off the calendar entry. for example :
Subject : <client> - <Short description of the job done>
And on the body part :
Project : <project name>
Task : <task name>
Billable : <yes or no> (this line can also be computed directly from the project name for example)
Comments : <more desciption, will not be printed on the standard reports>