>depend on inputs that are merely convenient side-effects of third-party systems provided free of charge and without obligation.
I would disagree. You don't get it for free, you pay for that. The Grid frequency is regulated and the powergrid providers have, IIRC, even legal obligations on how far they are allowed to deviate the time.
Using AC as clock signal is good enough if being wrong by a few minutes is not mission critical. If you absolutely do need accurate time use DCF77 or GPS.
I would disagree. You don't get it for free, you pay for that. The Grid frequency is regulated and the powergrid providers have, IIRC, even legal obligations on how far they are allowed to deviate the time.
Using AC as clock signal is good enough if being wrong by a few minutes is not mission critical. If you absolutely do need accurate time use DCF77 or GPS.