[7kay]

This reminded me of ledger, the plain text accounting tool. FWIW, ledger has time tracking and billing capabilities, too.

For example: https://github.com/anarcat/ledger-timetracking

You can also use the aforementioned org-clock-in with it.

[anarcat]

"author" of ledger-timetracking here, AMA, although I don't use it anymore...