55

I don't have to set the clock (besides the timezone) on the Raspberry Pi. Where is it getting the time from? About how accurate is it?

Improve this question
1

6 Answers 6

Reset to default
50

Raspbian gets the time from an NTP Server (a "time server").

Unplug your Raspberry Pi completely, pull out the network cable and start the Raspberry Pi up again. You will see that the date and time are incorrect.

If you want/need the date and time to not be reset without being plugged into the internet, you'll need a Real Time Clock (RTC) - for example, something like this - which will keep the time current, by using a battery to run a clock. This is, also, how your computer keeps the time when it is not being powered.

UPDATE Just a side note that the above RTC clock is not the only way to keep the time accurate between power losses. You could also use a GPS module and hook that up to the GPIO pins.

Accuracy It's pretty accurate, although if your clock's time is off by quite a bit. It may take up to 3 hours to correct itself, as changes from the server are applied gradually to your local clock. Network stability has the biggest impact on the accuracy, as a unpredictable network (something more wireless, like 3G) will make it very hard to be accurate. To put it simply: The accuracy is pretty good, the time difference between your computer and the actual time (from the NTP Server) is normally less than 100ms.

Improve this answer
8
  • 1
    nice answer. thanks for the link about hardware clock
    – Scoop
    Commented Jan 7, 2013 at 21:47
  • 1
    Nice answer; it could be improved by mentioning the accuracy of the NTP protocol.
    – Alex Chamberlain
    Commented Jan 9, 2013 at 13:02
  • @AlexChamberlain Thanks for the heads up, I was going to add it but it completely slipped my mind. I've added it now.
    – Vincent P
    Commented Jan 10, 2013 at 5:41
  • While my Pi is network connected, I'm noticing that time has drifted by 30mins now. Is the ntpd enabled by default or do I need to do this manually?
    – andig
    Commented Jun 6, 2013 at 10:41
  • Maybe so, until Raspberry Pi 3. I bought one two months ago and NTP time update is screwed, well at least on official Raspbian Jessie. I opened three threads with that problem and everyone are clueless. I had to buy hardware module for time keeping to solve the problem.
    – Pygmalion
    Commented May 23, 2016 at 7:47
22

Morgan's answer gave me the hint I needed to solve the problem.

  1. Go to http://support.ntp.org/bin/view/Servers/NTPPoolServers
    Select your region, then your country and a list of servers will be display.

  2. Edit your /etc/ntp.conf file e.g.

    sudo vim /etc/ntp.conf

    Replace the list of servers with the one you found in the webpage. e.g.

    server 0.dk.pool.ntp.org iburst
    server 1.dk.pool.ntp.org iburst
    server 2.dk.pool.ntp.org iburst
    server 3.dk.pool.ntp.org iburst

    Save and exit.

  3. Restart the ntp deamon.

    sudo /etc/init.d/ntp restart

  4. check that the date is correct.

    date
    Thu Jun 20 13:39:20 CEST 2013

In one of my servers the date was changed inmediately. In the other I had to wait a couple of minuts.

Improve this answer
6
  • 2
    If you're in the UK (or another country) then you can just change the two letter country code to suit .e.g. server 0.uk.pool.ntp.org iburst
    – Pierz
    Commented Apr 13, 2015 at 12:05
  • Step 4 did not work for me until after about 15 minutes. Or perhaps because I ran ntpq -p. I am using: "Raspbian GNU/Linux 7 (wheezy)" and "Linux raspberrypi 4.1.7+ #817 PREEMPT Sat Sep 19 15:25:36 BST 2015 armv6l GNU/Linux"
    – Peter Mortensen
    Commented Nov 26, 2016 at 14:47
  • Perhaps hwclock or fake-hwclock is interfering (but the "/etc/rc.conf" part is out of date)
    – Peter Mortensen
    Commented Nov 26, 2016 at 14:57
  • The ntpq -p part is from an answer to How is time kept on the Raspberry Pi?.
    – Peter Mortensen
    Commented Nov 26, 2016 at 15:11
  • And after a 5 minute shutdown it was 5 minutes behind (ntpq -p did not make a difference).
    – Peter Mortensen
    Commented Nov 26, 2016 at 15:25
19

If you have the fake-hwclock package, it will also do a timestamp saving trick to prevent the clock from being reset. That won't advance time while the device is off, so it will lag behind. The internal clock will also drift a tiny bit while on.

If you have an Internet connection (or lan with a server) and run ntp (ntpd/ntpdate), you can get the time over network, as other answers point out. When synchronized, it is really quite accurate. Online servers will also give you the consensus "world time".

Improve this answer
6
  • interesting about fake-hwclock. Why would someone want to freeze the clock when the machine is off? I can't think of a use case for that. Is there?
    – Scoop
    Commented Jan 11, 2013 at 22:44
  • 2
    @Scoop Because the alternative is forgetting the time, so it will reset. This will cause any recorded timestamps appear to be from the future etc. "Freezing" the clock makes sure time keeps moving forward, even if the count is off :)
    – XTL
    Commented Jan 14, 2013 at 8:03
  • 1
    Having ntp or other way to set the clock of course will still work and fake-hwclock only runs at boot and shutdown.
    – XTL
    Commented Jan 14, 2013 at 8:08
  • 1
    for fake-hwclock on Arch Linux, use archplusplus.co.uk/post/40202081414/…
    – levinalex
    Commented Apr 8, 2013 at 12:19
  • 2
    This package now seems to be installed as default on Raspbian now.
    – Pierz
    Commented Apr 14, 2015 at 7:59
12

The Raspberry Pi doesn't have an integrated real time clock (RTC) for cost-effectiveness: see the question "Why is there no real time clock (RTC)?" in the official FAQ. It means that when you unplug the Raspberry Pi, it loses its time. When you power it back, the time is initialized to 1 January 1970 (aka Unix epoch).

The Raspberry Pi sets its time over the network with NTP, a protocol for clock synchronization between computers. This protocol is widely used over the Internet to make sure the computers have the same time and is highly reliable since some machines are dedicated to the time calculation with atomic clocks.

ntpd is a daemon (a software in background) launched a boot time, and it is responsible for synchronizing time using the NTP protocol. ntpd can be configured with the file /etc/ntp.conf. For instance, this is where you can set the servers to request the time.

Improve this answer
3
  • Doesn't it remember the last time? I shutdown a Raspberry Pi a week ago and date now shows that week-old date plus, I think, some extra hours from the uptime yesterday and today (after some restarts yesterday and today). (Raspbian, corresponding to mid 2015).
    – Peter Mortensen
    Commented Dec 11, 2015 at 14:46
  • Run cat /etc/fake-hwclock.data and you will see the last time fake-hwclock saved the time (obviously, at shutdown time). fake-hwclock is used on the RPi to mimic a real clock. I think you booted your RPi without Internet access or without ntpd started to update the time. Can you confirm that?
    – Morgan Courbet
    Commented Dec 11, 2015 at 15:00
  • On power-on the Raspberry Pi has a date/time of zero. By definition zero is Jan 1, 1970 UTC. Linux systems keep track of UTC time and convert it to a local timezone for display. Note that US timezone have negative time zone offsets, so zero UTC is displayed in (-7:00) Arizona as 17:00 Dec 31, 1969. In the 21 century, a date of 1970 is obviously wrong. A number of schemes exist for setting an incorrect date/time that is closer to correct than 1970. Correct time is normally set by getting current date/time from an NTP server on the Internet, or from a local hardware clock (RTC/GPS).
    – Chad Farmer
    Commented Oct 5 at 16:44
11

Raspbian Stretch: NTP not installed by default

It would appear that there was a bit of change-up with Raspbian Stretch. In short, ntp is no longer installed. Running dpkg -l | grep ntp returns nothing.

Apparently, Stretch abandoned the ntp application in favor of systemd. Check the man timesyncd entry for specifics on configuration settings. timesyncd still uses ntp servers to update the time. If you want to configure these servers manually, you can do so in the /etc/systemd/timesyncd.conf file.

Improve this answer
2
  • 1
    After doing the edit, restart the service by sudo systemctl restart timesyncd and after that check the logs by journalctl -u timesyncd -f sometimes corporate firewalls blocks the port. In such cases, use the provided corporate ntp server URL.
    – daparic
    Commented Feb 5, 2019 at 19:01
  • 1
    I believe the correct name of the daemon is systemd-timesyncd, so that the command to restart would be systemctl restart systemd-timesyncd
    – chb
    Commented Aug 2, 2019 at 16:49
3

If logging or keeping time when offline is what you're looking for, you may want to add a Real Time Clock chip, like the DS1307.

Improve this answer
1

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.