It can be difficult to know which hours have been invoiced and which ones haven't. Timely solves this by letting you mark hours as billed.
If you open a project, you'll see all your tasks in a timeline view. On the side you'll have check boxes to mark time as billed or unbilled. Check off the tasks you want to mark, and they'll get a separate look with a new icon to differentiate them from the rest. You'll be able to see the total time billed or unbilled.
Note: You can't filter by billed or unbilled on the Reports page just yet, but this will be coming later.