We offer an online service using OpenTimeTool on our servers. The time tracking software makes it possible to account for any time spent on a project. 

You simply enter a start time and assign a project, once you are done with the task you enter a stop time or just hit the break or stop buttom. Your time is booked onto the project.  For each time booking you can also enter a text on what was done.

As an administrator you can enter/edit new projects or clients and assign a team with team leader and other members that can book time onto the project.

That way you can work from anywhere in the world and always have an overview on working hours spent on each project. You can pull several reports out of the system and once the project is done you can close it and send your invoice with an exact amount of time spent per day including comments on the work done.