About time synchronisation
Network Time Protocol (NTP) is a networking protocol for synchronising time over a network. Basically, a client requests the current time from a server, and uses it to set its own clock.
Behind this simple description, there is a lot of complexity. There are three tiers of NTP servers; tier one NTP servers are connected to atomic clocks, while tier two and tier three three servers spread the load of actually handling requests across the Internet.
The client software is also a lot more complex than you might expect. It must factor in communication delays and adjust the time in a way that does not upset all the other processes that run on the server. Luckily, all that complexity is hidden from you!
By default, Ubuntu uses timedatectl/timesyncd to synchronise time, and they are available by default. See our guide If you would like to know how to configure timedatectl and timesyncd.
Users can also optionally use chrony to serve NTP.
How time synchronisation works
Since Ubuntu 16.04, timedatectl/timesyncd (which are part of systemd) replace most of ntpdate/ntp.
About timesyncd
timesyncd replaces not only ntpdate, but also the client portion of chrony (formerly ntpd). So, on top of the one-shot action that ntpdate provided on boot and network activation, timesyncd now regularly checks and keeps your local time in sync. It also stores time updates locally, so that after reboots the time monotonically advances (if applicable).
About timedatectl
If chrony is installed, timedatectl steps back to let chrony handle timekeeping. This ensures that no two time-syncing services can conflict with each other.
ntpdate is now considered deprecated in favor of timedatectl (or chrony) and is no longer installed by default. timesyncd will generally keep your time in sync, and chrony will help with more complex cases. But if you had one of a few known special ntpdate use cases, consider the following:
- If you require a one-shot sync, use:
chronyd -q - If you require a one-shot time check (without setting the time), use:
chronyd -Q
While use of ntpd is no longer recommended, this also still applies to ntpd being installed to retain any previous behaviour/config that you had through an upgrade. However, it also implies that on an upgrade from a former release, ntp/ntpdate might still be installed and therefore renders the new systemd-based services disabled.
Further reading
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Also see the Ubuntu Time wiki page for more information.