Linux ntp

Declaimer: Most these commands have been used on Ubuntu systems.

Display time details (Ubuntu 16.04)

$ timedatectl
      Local time: Tue 2017-08-08 09:31:14 UTC
  Universal time: Tue 2017-08-08 09:31:14 UTC
        RTC time: Tue 2017-08-08 09:31:14
       Time zone: Etc/UTC (UTC, +0000)
 Network time on: yes
NTP synchronized: no
 RTC in local TZ: no

Set timezone (Ubuntu 16.04)

sudo timedatectl set-timezone Europe/London

New NTP packages

sudo update-rc.d -f ntpdate remove     -remove deprecated ntpdate service
sudo apt-get install ntp

Show ntp logs

cat /var/log/messages | grep ntp

Show RTC (Real Time Clock) hardware clock

sudo hwclock --show  
hwclock -r
hwclock --show --utc

Sync system clock -> hardware clock

sudo hwclock --systohc

Sync hardware clock -> system clock

hwclock --hctosys

Troubleshooting NTP server

sudo tcpdump port 123    #enable ntp packet logging
timedatectl status    
ntpdc -p     #offset in seconds
ntpq -p      #offset in milliseconds

ntpq --numeric --peers   #show peers (time providers)
ntpq -c lpeer            #show peers (time providers)
ntpq -d <time_server>       #debugging mode (not sure if will update local clock)
ntpdate -d <time_server>    #debugging mode, in which ntpdate will go through all the steps, but doesn't adjust the local clock
ntpstats
ntpdate -s a_stratum_1_server_address

ntpdc -c loopinfo   #will display the combined offset in seconds, as seen at the last poll
ntpdc -c kerninfo   #will display the current remaining correction, just as ntptime does

Show difference/offset between local and remote time server

ntpdate -q <time_server>    #just query NTP server and it will display time difference/offset

References

Ntp Ubuntu wiki