Unbilled Hours

I use a few basic Projects within Zoho Invoice. I have created an invoice for a 25 hour project and mailed the invoice...it's one line item for the entire project. Is there a way to set the "Billed Hours" to 25 so that I know I've already invoiced the client for the full project? Otherwise the time tracking screen implies that every amount of time I apply to the project is unbilled.

Thanks!