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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?

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5 Answers

up vote 22 down vote accepted

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.

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nice answer. thanks for the link about hardware clock –  Scoop Jan 7 '13 at 21:47
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Nice answer; it could be improved by mentioning the accuracy of the NTP protocol. –  Alex Chamberlain Jan 9 '13 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 Jan 10 '13 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 Jun 6 '13 at 10:41
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The Raspberry Pi doesn't have 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 RPi, it loses its time. When you power it back, the time is initialized to 1 January 1970 (aka Unix epoch).

The RPi 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 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.

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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".

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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 Jan 11 '13 at 22:44
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@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 Jan 14 '13 at 8:03
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Having ntp or other way to set the clock of course will still work and fake-hwclock only runs at boot and shutdown. –  XTL Jan 14 '13 at 8:08
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for fake-hwclock on Arch Linux, use archplusplus.co.uk/post/40202081414/… –  levinalex Apr 8 '13 at 12:19
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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.

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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.

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