As you can see in the section above, you can enter tasks at any time while using phpaga. At the time you record a task, you can set a duration, or a start and stop time, plus a description. At that time you specify the project to which the task belongs, but you *DON'T* specify the invoice. Again, seemingly complex but you will find that it is really FLEXIBILITY in disguise. You may have worked some hours in May but you want, or the client asked you to bill them on your June invoice. That's no problem.
When you add tasks they go into a pool of unassigned hours. Under the FINANCE section you can click "Unbilled hours" at any time to see the list. Of course when you go to create an invoice, and assign those tasks, they will no longer be included in this list.