$ sudo ntpdate org 31 Aug ntpdate: the NTP socket is in use, exiting $ sudo service ntp stop [ ok ] Stopping NTP server: ntpd.$ sudo ntpdate org 31 Aug ntpdate: adjust time server .115 offset -0.002893 sec $ sudo service ntp start For people reading the way above, though it technically works, look below at superuser.com/a/639516/308927 because it's way simpler (single command, no need to twiddle with stopping/starting the service).
Behind this simple description, there is a lot of complexity - there are tiers of NTP servers, with the tier one NTP servers connected to atomic clocks, and tier two and three servers spreading the load of actually handling requests across the Internet.
Also the client software is a lot more complex than you might think - it has to factor out communication delays, and adjust the time in a way that does not upset all the other processes that run on the server. That shall ensure that no two time syncing services are fighting and also to retain any kind of old behaviour/config that you had through an upgrade.
But luckily all that complexity is hidden from you! But 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.
timedatectl status Local time: Fri 2016-04-29 UTC Universal time: Fri 2016-04-29 UTC RTC time: Fri 2016-04-29 Time zone: Etc/UTC (UTC, +0000) Network time on: yes NTP synchronized: no RTC in local TZ: no and thereby no more installed by default.
If installed it will run once at boot time to set up your time according to Ubuntu's NTP server.
Later on anytime a new interface comes up it retries to update the time - while doing so it will try to slowly drift time as long as the delta it has to cover isn't too big.That behaviour can be controlled with the By default the systemd based tools request time information at ntp.In classic ntpd based service uses the pool of [0-3].Of the pool number 2.as well as ntp.also support ipv6 if needed.If one needs to force ipv6 there also is ipv6which is not configured by default. The ntp daemon ntpd calculates the drift of your system clock and continuously adjusts it, so there are no large corrections that could lead to inconsistent logs for instance. The cost is a little processing power and memory, but for a modern server this is negligible. Approved by Ubuntu Technical Board # on 2011-02-08 (LP: #104525). server 0.server 1.server 2.server 3.remote refid st t when poll reach delay offset jitter ============================================================================== +stratum2-2. 1.70 2 u 5 64 377 68.461 -44.274 110.334 +ntp2.m-online.n 184.108.40.206 2 u 5 64 377 54.629 -27.318 78.882 *145.2 .