The Network Time Protocol (NTP) is a network protocol for clock synchronization between computer systems. NTP timestamps are sent and received using the User Datagram Protocol (UDP) on port number 123.
Public Time Servers
Enterprise IT Architectures often have local time servers to synchronize time across on-premise servers. If no local NTP Server is available, public time servers are a viable option.
NTP Pool Project
The NTP Pool Project provides a cluster of timeservers providing reliable easy to use NTP service for millions of clients.
Google Public NTP
Google Public NTP is a free, global time service that can be used to synchronize to Google's atomic clocks.
Cloudflare’s Time Service
Cloudflare’s Time Service is a free time service that supports both NTP and the emerging Network Time Security (NTS) protocol for securing NTP.