As suggested before - Add a timer for billing options

There are many billing, invoicing, and time tracking software(s) for Windows in the Windows store. You may get more traction if you can find an industry standard that has an API or SDK. Then TeamViewer folks don't have to re-invent the wheel, which is really what you are asking for.

I use the time stamps on the session chat to both log the session, and track the billable clock time.

This chat can be saved from the little side panel via rt-click, Select All, Copy, then Pasted into Notepad or whatever. Alas, it can't be saved directly as a file, nor does it exist as a file anywhere that I can find.

But TeamViewer has a fatal flaw; it throws away this text chat whenever the session ends for any reason - so everything is lost. Seems crazy to me, and I'm advocating where appropriate to have this fixed!

I too would appreciate having some sort of timesheet, though not necessarily a billing service. One simple solution would be having a separate log for sessions (host details, date-time started, session length) which could be enabled in the options window. Once enabled, every session would be recorded and those of us who need the information, could collect it from the file at our convenience.