Meta's time service now lets devices verify the clock, not just read it
NTS: Authenticated Time at Meta
Meta has added Network Time Security (RFC 8915) to its public time service at nts.meta.com, so packets are authenticated and devices can confirm the time came from Meta unmodified. The servers keep no per-client state: cookie keys are derived from a shared master secret and the current day, avoiding replication. Meta open-sourced the protocol, server, and client in its Time library on GitHub. It urges NTP client maintainers on Android and iOS to enable NTS support.
Authentication tells you who wrote a packet, not how long it spent in flight.