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

Output
Tue 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 timedatectl command 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

Output
Tue 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.