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
timesyncd to synchronise time, and they are available by default. See our guide If you would like to know how to configure
Users can also optionally use
chrony to serve NTP.
How time synchronisation works
Since Ubuntu 16.04,
timesyncd (which are part of
systemd) replace most of
timesyncd replaces not only
ntpdate, but also the client portion of
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).
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
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:
- If you require a one-shot time check (without setting the time), use:
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,
ntpdate might still be installed and therefore renders the new
systemd-based services disabled.