Accuracy is estimated from HTTPS timing measurements: each of the ~10 exchanges records client send, server receive, server send and client receive; the lowest-delay half is kept and the median offset used. The uncertainty is (min delay)/2 + sample spread + 1 ms and assumes a symmetric network path. Network-path asymmetry cannot be measured and may introduce additional error, so this is an estimate, not a bound, and not NTP-grade accuracy. It also says nothing about audio timing or the device's audio sample clock.