Linux ntp

Most these commands have been used on Ubuntu systems.

Time manipulation and troubleshooting

timedatectl - In recent Ubuntu releases timedatectl replaces ntpdate. By default timedatectl syncs the time once on boot and later on uses socket activation to recheck once network connections become active.

timesyncd - In recent Ubuntu releases timesyncd replaces the client portion of ntpd. By default timesyncd regularly checks and keeps the time in sync. It also stores time updates locally, so that after reboots monotonically advances if applicable.

Update NTP managment system

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

hwclock --show  
hwclock -r
hwclock --show --utc
hwclock --systohc #sync system clock -> hardware clock
hwclock --hctosys #sync hardware clock -> system clock

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 time zone

Set timezone (Ubuntu 16.04)

sudo timedatectl set-timezone Europe/London

Set time zone on Amazon Linux, to UTC

cat /etc/*rel*
NAME="Amazon Linux AMI"
VERSION="2014.09"

$ date
Fri  4 Sep 15:32:12 BST 2020

# Do not change the 'UTC=true' entry to another value. This entry is for the hardware clock, 
# and does not need to be adjusted when you're setting a different time zone on your instance.
sudo vi /etc/sysconfig/clock # edit if needed
ZONE="UTC"
UTC=true

# List available time zones
ls -l /etc/localtime

# Symlink (cp) 'localtime' file with content of desired TZ
sudo mv /etc/localtime /etc/localtime.original
sudo ln -s /usr/share/zoneinfo/UTC /etc/localtime

date # no reboot required
Fri  4 Sep 14:50:26 UTC 2020

chrony

Every pre-RHEL8 uses ntpd but recently chrony becomes a standard time client.

• Setting the time for your Linux instance AWS docs

# RHEL8
sudo yum -y install chrony
sudo systemctl start chronyd && sudo systemctl enable chronyd
sudo vi /etc/chrony.conf # add NTP server, service restart is required to apply changes

Show time sources

chronyc sources -v
  .-- Source mode  '^' = server, '=' = peer, '#' = local clock.
 / .- Source state '*' = current synced, '+' = combined , '-' = not combined,
| /   '?' = unreachable, 'x' = time may be in error, '~' = time too variable.
||                                                 .- xxxx [ yyyy ] +/- zzzz
||      Reachability register (octal) -.           |  xxxx = adjusted offset,
||      Log2(Polling interval) --.      |          |  yyyy = measured offset,
||                                \     |          |  zzzz = estimated error.
||                                 |    |           \
MS Name/IP address         Stratum Poll Reach LastRx Last sample
===============================================================================
^* 169.254.169.123               3   6    17    12    +15us[  +57us] +/-  320us
^- tbag.heanet.ie                1   6    17    13  -3488us[-3446us] +/- 1779us
^- ec2-12-34-231-12.eu-west-     2   6    17    13   +893us[ +935us] +/- 7710us
^? 2a05:d018:c43:e312:ce77:6     0   6     0   10y     +0ns[   +0ns] +/-    0ns
^? 2a05:d018:d34:9000:d8c6:5     0   6     0   10y     +0ns[   +0ns] +/-    0ns
^? tshirt.heanet.ie              0   6     0   10y     +0ns[   +0ns] +/-    0ns
^? bray.walcz.net                0   6     0   10y     +0ns[   +0ns] +/-    0ns

References

Ntp Ubuntu wiki