3

The Raspberry Pi has no hardware clock, so the OS has to fetch the date and time every time when it boots if one wants accurate time.

Do I need accurate NTP time at all and what use cases require it?

I'm using Raspbian, but I assume this question holds for any distribution.

  • How accurate is 'accurate'? Do you not want your system logs to show the correct time and date? Do you want your files and folders to have the correct creation and modified timestamps? Do you want to be able to update firmware with rpi-update? – drgrog May 7 '14 at 15:36
  • With "accurate" I mean to the second, give or take some (not millisecond accurency or something). – Foo Bar May 7 '14 at 16:04
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Depends on what you use your Pi for.

If you use it to develop, like I do, and for that use git or some other distributed version control system, it gets pretty annoying to have a new commit dated 3 days ago.

If you need public-key infrastructure certificates, that have begin and end times on them, you will have expired or not-yet-valid errors at some point.

If you run a backup application, that needs to schedule when to suck data from your network and store it elsewhere, you will need accurate time to make sure you won't do that right in the middle of the day, where open files are everywhere and network bandwidth might be affected.

But yes, for all those reasons NTP isn't necessary. You can setup a rdate or ntpdate on boot and not have ntp trying to compensate for clock fluctuations of your processor, at you will do just fine.

But there is at least one use that I can think of that will need NTP-level accuracy: parallel and distributed use and processing. Be it a load balanced web server, be it a file storage system, be it a file mirroring task.

For most of the other cases, you can just replace NTP with something else, like I suggested earlier, rdate or even ntpdate. The latter seems to be easier to get going these days, given that NTP is pretty standard and rdate protocol ports are almost certainly filtered or closed everywhere around.

I have had one Pi running unsynced for some time, on a air gap security application. The experience was awful, even if the application did not need any sort of time-accuracy. I would get crazy trying to figure out the creation or change dates of files, trying to figure out information from logs (when did that happen?!) and all sorts of things that you usually place in a window of time to understand or analyse.

1

you'll definitely need accurate time if you use raspberry pi to serve web pages over SSL connection, has something to do with the certificate expiration and generally may cause a lot of grief.

besides that, I would imagine, if you run any web-server at all, you might want to have your timestamps right for all kind of caches, that rely on expiration date/time, to work properly.

finally, if you have any VCS system (git, svn, mercurial) installed or used, it might be better if the time is correct, to avoid "the file from the future, that overwrites everything" kind of situations.

0

Answer: You always want the most accurate time possible, since it is so easy to do it.

For instance, you will note that all fresh Windows installs and Linux installs utilize pools of servers that get there time from elsewhere and pass it on to you.

To get the actual Master Clock time.nist.gov is easy. It is what they call Stratum-1 which means it is the best you can get on the Internet.

The procedure to set it up is very easy, as explained in this Q&A on this Stack Exchange:

How do I set Raspbian to use the primary time server time.nist.gov?

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