Track time without a project

You don't need a project to track time. Start the timer first and sort it out later, or never, if the work isn't for anyone in particular.

Your choices in the project menu

Open the project button on the timer bar, the manual form or any entry:

No project — the entry isn't filed under anything.
A client, then No project — bill to the client — the time belongs to that client but to none of their projects.
A client, then one of their projects — the usual case.
Internal — projects that don't belong to any client, such as admin or business development. Only shown if your workspace has some.

the project menu open on a client, showing 'No project — bill to the client'

What happens to the time

No project, no client: the time counts on your timesheet and in reports, but it can't go on an invoice, because invoices are raised for a client. File it under a client or project when you know where it belongs.
Client only: the time is billed to that client like any other. If they have hours on a retainer, it draws those down; otherwise it's billed at the rate. It goes on an invoice made for the whole client, not on one made for a single project. It doesn't appear in the client's portal, which only shows time on projects you've made visible to them.

Filing it later

Click the project button on the entry and pick the right project. Timebun re-prices the entry for its new home. Invoiced entries are locked and can't be moved.

When a project is required

If the Owner (shown as Portal admin in the app for now) turns on Require a project on every entry (Settings → Time tracking), the No project choices disappear. The project button reads Choose a project, and a timer won't start without one. Entries tracked before the switch keep their empty project and can still be edited; you only have to pick a project if you change where one is filed.