NTP

The Network Time Protocol (NTP) is a networking protocol for clock synchronization between computer systems over packet-switched, variable-latency data networks. NTP is intended to synchronize all participating computers to within a few milliseconds of Coordinated Universal Time (UTC). NTP can usually maintain time to within tens of milliseconds over the public Internet, and can achieve better than one millisecond accuracy in local area networks.

Implementations send and receive timestamps using the User Datagram Protocol (UDP) on port number 123.

NTP servers use "UTC" time (formerly "GMT" time), which is the same throughout the world. The timezone you are in doesn't matter to this NTP server. The translation to a time zone is handled completely by your NTP client software.

Variations

The NTP server uses UDP port 123 ("ntp"). Other time clients use other time protocols. That is, port 13 ("daytime"), 37 ("time"), or 525 ("timed").

Simple Network Time Protocol (SNTP) is a less complex implementation of NTP, using the same protocol but without requiring the storage of state over extended periods of time.

W32Time is a bastardized variation which has the ability to synchronize the computer clock to an NTP server. Windows 2000 and Windows XP only implements SNTP, and violates several aspects of the NTP. Microsoft states that W32Time cannot reliably maintain time synchronization with one second accuracy

Test NTP Service from Linux CLI

sudo apt install ntpdate
ntpdate -q time.google.com

Installs the utility. The -q command switch tells ntpdate not to sync the system clock, only show results of query as well as it compares to your system clock.