Ledger has some built in: https://www.ledger-cli.org/3.0/doc/ledger3.html#Time-Keeping

There's also Timewarrior from the Taskwarrior people: https://timewarrior.net/

Would using ledger & friends, with “hours” (and, with preset exchange rates, minutes and seconds) as commodities work for your use case?

That was my first thought, but my brain loves reinventing wheels.

hledger has provisions and advice on the docs (iirc) for time accounting