I’m not sure that deploying a custom clock synchronisation system would be warranted to show a countdown timer on the screen. The fix is to accept that the interface may be potentially inaccurate by a few seconds and send the relative time rather than the expiry time. Also probably to make the expiry UI stable (ie. have a “try again” button rather than flipping back to the start screen when the timer expires).

I also think that expecting that a computer connected to the internet in 2026 has the correct time isn’t too much of a long shot and things like 2fa QR codes rely on correct timing anyway.