Debian 20

You can use Linux as both an NTP client or server. NTP is installed through:apt-get install ntp

NTP uses port 123. It synchronizes clocks and relies upon tiered systems (strata). You can sync with numerous NTP servers as well.

NTP is configured in:
/etc/ntp.conf

To return the queue of NTP clocks:
ntpq -p

+ means usable as primary clock, and * means usable as secondary clock.

There are 16 strata, 1-16 where 1 is the most accurate (atomic clock). Each level of sync will increase the strata by 1. Define at least 3 NTP time soures for accuracy.

To force an NTP update, use:
ntpd -g

To add NTP servers, modify /etc/ntp.conf like this:
server 0.us.pool.ntp.org
server 1.us.pool.ntp.org
server 2.us.pool.ntp.org