Introduction
Accurate timekeeping has become a critical component of modern software deployments. Whether it's making sure logs are recorded in the right order or database updates are applied correctly, out-of-sync time can cause errors, data corruption, and other hard to debug issues.
Debian 9 has time synchronization built in and activated by default using the standard ntpd time server, provided by the ntp package. In this article we will look at some basic time-related commands, verify that ntpd is active and connected to peers, and learn how to activate the alternate systemd-timesyncd network time service.
Prerequisites
Before starting this tutorial, you will need a Debian 9 server with a non-root, sudo-enabled user, as described in this Debian 9 server setup tutorial.
Navigating Basic Time Commands
The most basic command for finding out the time on your server is date. Any user can type this command to print out the date and time:
date
OutputTue Sep 4 17:51:49 UTC 2018
Most often your server will default to the UTC time zone, as highlighted in the above output. UTC is Coordinated Universal Time, the time at zero degrees longitude. Consistently using Universal Time reduces confusion when your infrastructure spans multiple time zones.
If you have different requirements and need to change the time zone, you can use the timedatectlcommand to do so.
First, list the available time zones:
timedatectl list-timezones
A list of time zones will print to your screen. You can press SPACE to page down, and b to page up. Once you find the correct time zone, make note of it then type q to exit the list.
Now set the time zone with timedatectl set-timezone, making sure to replace the highlighted portion below with the time zone you found in the list. You'll need to use sudo with timedatectl to make this change:
sudo timedatectl set-timezone America/New_York
You can verify your changes by running date again:
date
OutputTue Sep 4 13:52:57 EDT 2018
The time zone abbreviation should reflect the newly chosen value.
Now that we know how to check the clock and set time zones, let’s make sure our time is being synchronized properly.
Checking the Status of ntpd
By default, Debian 9 runs the standard ntpd server to keep your system time synchronized with a pool of external time servers. We can check that it's running with the systemctl command:
sudo systemctl status ntp
Output● ntp.service - LSB: Start NTP daemon
Loaded: loaded (/etc/init.d/ntp; generated; vendor preset: enabled)
Active: active (running) since Tue 2018-09-04 15:07:03 EDT; 30min ago
Docs: man:systemd-sysv-generator(8)
Process: 876 ExecStart=/etc/init.d/ntp start (code=exited, status=0/SUCCESS)
Tasks: 2 (limit: 4915)
CGroup: /system.slice/ntp.service
└─904 /usr/sbin/ntpd -p /var/run/ntpd.pid -g -u 105:109
. . .
The active (running) status indicates that ntpd started up properly. To get more information about the status of ntpd we can use the ntpq command:
ntpq -p
Output remote refid st t when poll reach delay offset jitter
==============================================================================
0.debian.pool.n .POOL. 16 p - 64 0 0.000 0.000 0.000
1.debian.pool.n .POOL. 16 p - 64 0 0.000 0.000 0.000
2.debian.pool.n .POOL. 16 p - 64 0 0.000 0.000 0.000
3.debian.pool.n .POOL. 16 p - 64 0 0.000 0.000 0.000
-eterna.binary.n 204.9.54.119 2 u 240 256 377 35.392 0.142 0.211
-static-96-244-9 192.168.10.254 2 u 60 256 377 10.242 1.297 2.412
+minime.fdf.net 83.157.230.212 3 u 99 256 377 24.042 0.128 0.250
*t1.time.bf1.yah 98.139.133.62 2 u 31 256 377 11.112 0.621 0.186
+x.ns.gin.ntt.ne 249.224.99.213 2 u 108 256 377 1.290 -0.073 0.132
-ord1.m-d.net 142.66.101.13 2 u 473 512 377 19.930 -1.764 0.293
ntpq is a query tool for ntpd. The -p flag asks for information about the NTP servers (or peers) ntpd is connected to. Your output will be slightly different, but should list the default Debian pool servers plus a few others. Bear in mind that it can take a few minutes for ntpd to establish connections.
Switching to systemd-timesyncd
It is possible to use systemd's built-in timesyncd component to replace ntpd. timesyncd is a lighter-weight alternative to ntpd that is more integrated with systemd. Note however that it doesn't support running as a time server, and it is slightly less sophisticated in the techniques it uses to keep your system time in sync. If you are running complex real-time distributed systems, you may want to stick with ntpd.
To use timesyncd, we must first uninstall ntpd:
sudo apt purge ntp
Then, start up the timesyncd service:
sudo systemctl start systemd-timesyncd
Finally, check the status of the service to make sure it's running:
sudo systemctl status systemd-timesyncd
Output● systemd-timesyncd.service - Network Time Synchronization
Loaded: loaded (/lib/systemd/system/systemd-timesyncd.service; enabled; vendor preset: enabled)
Drop-In: /lib/systemd/system/systemd-timesyncd.service.d
└─disable-with-time-daemon.conf
Active: active (running) since Tue 2018-09-04 16:14:23 EDT; 1s ago
Docs: man:systemd-timesyncd.service(8)
Main PID: 3399 (systemd-timesyn)
Status: "Synchronized to time server 198.60.22.240:123 (0.debian.pool.ntp.org)."
Tasks: 2 (limit: 4915)
CGroup: /system.slice/systemd-timesyncd.service
└─3399 /lib/systemd/systemd-timesyncd
We can use timedatectl to print out systemd's current understanding of the time:
timedatectl
Output Local time: Tue 2018-09-04 16:15:34 EDT
Universal time: Tue 2018-09-04 20:15:34 UTC
RTC time: Tue 2018-09-04 20:15:33
Time zone: America/New_York (EDT, -0400)
Network time on: yes
NTP synchronized: yes
RTC in local TZ: no
This prints out the local time, universal time (which may be the same as local time, if you didn't switch from the UTC time zone), and some network time status information. Network time on: yes means that timesyncd is enabled, and NTP synchronized: yes indicates that the time has been successfully synced.
Conclusion
In this article we’ve shown how to view the system time, change time zones, work with ntpd, and switch to systemd's timesyncd service. If you have more sophisticated timekeeping needs than what we’ve covered here, you might refer to the offical NTP documentation, and also take a look at the NTP Pool Project, a global group of volunteers providing much of the world's NTP infrastructure.
