Timeslips are nothing to do with charitable status, some charities might record volunteer hours so they can record a “ donation in kind” or similar. Sometimes funders of charities have match funding requirements whereby the recipient charity has to demonstrate that it has raised funding from other sources. As jws don’t take institutional charitable funding then this isn’t an issue.
Jw’s definitely meet the definition of charity (at least in the uk), as they have a remit and objective to educate and publish educational material without charge.