How do Raspberry Pis know what the time is?

On my Pis NTP isn't installed but they seem to know what the time is
OK so how do they do it?

--
Chris Green
·

On 2/10/22 15:07, Chris Green wrote:
> How do Raspberry Pis know what the time is?
>
> On my Pis NTP isn't installed but they seem to know what the time is
> OK so how do they do it?
>

systemd.timesync.d

On 2/10/22 15:32, Knute Johnson wrote:
> On 2/10/22 15:07, Chris Green wrote:
>> How do Raspberry Pis know what the time is?
>>
>> On my Pis NTP isn't installed but they seem to know what the time is
>> OK so how do they do it?
>>
>
> systemd.timesync.d
>

too many .

systemd.timesyncd

On 2022-02-10, Chris Green <cl@isbd.net> wrote:
> How do Raspberry Pis know what the time is?
>
> On my Pis NTP isn't installed but they seem to know what the time is
> OK so how do they do it?
>

Depends what OS you've got. I'm assuming it is Raspberry Pi OS.

As far as I know systemd provides an NTP client - systemd-timesyncd

Try the command

systemctl status systemd-timesyncd

might need a sudo in front.

"Knute Johnson" <knute2020@585ranch.com> wrote in message
news:su409i$st3$1@dont-email.me...
> On 2/10/22 15:07, Chris Green wrote:
>> How do Raspberry Pis know what the time is?
>>
>> On my Pis NTP isn't installed but they seem to know what the time is
>> OK so how do they do it?
>>
>
> systemd.timesync.d

Beware of running time-sensitive software on a Pi that reboots and doesn't
immediately have an internet connection.

I use a Pi to run Cumulus weather station software. This logs and graphs
data from a weather station. If there has been a power cut, everything
*starts* to boot at the same time when the power comes back. If the Pi boots
first, and the router takes its time connecting, the Pi will run with a
silly time until it is able to synchronise with an internet time source.
This leads to graphs which suddenly shoot back by several hours (when the Pi
reboots) and then shoot forward again (once the Pi has managed to
synchronise). My impression is that the Pi checks frequently for a time
source to begin with, which is why I rarely see the problem, but then starts
polling less frequently, which is why on the rare occasions that I've seen
the problem, the update to the correct time happens a fair time after the
router has connected and I've been able to browse the web on other
computers. The Pi seems to have a crude real-time clock which gets the time
approximately right (maybe wrong by a couple of hours), rather than
reverting to "the beginning of time" (1 Jan 1970).

It's not the end of the world, and not worth the hassle of implementing a
proper battery-backed RTC (as on a Windows/Linux PC). The software logs
readings every 10 minutes (though it graphs them every minute) so there
aren't usually many readings that have an incorrect timestamp, and it's
obvious which times are missing because they *should* be at 00, 10, 20, 30,
40, 50 minutes past every hour. It's just a case of hand-editing the log
file and then restarting the software manually so it regenerates the graphs
with the correct time axis.

I imagine that a Pi that is run without an internet connection (eg an
unattended device with no Ethernet/wifi connection) will free-run - unless
it has a GPS connection (eg to record a track of where it is going) in which
case it can be configured to use the time from the GPS to sync the system
clock. I imagine there are RTC modules that communicate through the GPIO
socket, for devices that have no internet connection.

On Thu, 10 Feb 2022 21:07:55 +0000
Chris Green <cl@isbd.net> wrote:

> How do Raspberry Pis know what the time is?
>
> On my Pis NTP isn't installed but they seem to know what the time is
> OK so how do they do it?

Does it run ntpdate at boot ?

--
Steve O'Hara-Smith
Odds and Ends at http://www.sohara.org/

Re: How does Pi get its time?
By: Chris Green to All on Thu Feb 10 2022 09:07 pm

CG> How do Raspberry Pis know what the time is?

CG> On my Pis NTP isn't installed but they seem to know what the time is
CG> OK so how do they do it?

If you are running Raspbian, it could well be timesyncd.

Tristan.

Hello Chris!

Thursday February 10 2022 21:07, you wrote to All:

> How do Raspberry Pis know what the time is?

> On my Pis NTP isn't installed but they seem to know what the time is
> OK so how do they do it?

Are you SURE that it is not installed ?

Vincent

Vincent Coen <nospam.Vincent.Coen@f1.n250.z2.fidonet.org> wrote:
> Hello Chris!
>
> Thursday February 10 2022 21:07, you wrote to All:
>
> > How do Raspberry Pis know what the time is?
>
> > On my Pis NTP isn't installed but they seem to know what the time is
> > OK so how do they do it?
>
> Are you SURE that it is not installed ?
>
Well I can't see anything that looks like it:-

chris@backup$ ps -ef | grep -i ntp
chris 7885 7820 0 22:36 pts/0 00:00:00 grep --color=auto -i ntp
chris@backup$ man -k ntp
grantpt (3) - grant access to the slave pseudoterminal
intptr_t (3) - overview of system data types
mountpoint (1) - see if a directory or file is a mountpoint
ntp_adjtime (3) - tune kernel clock
ntp_gettime (3) - get time parameters (NTP daemon interface)
ntp_gettimex (3) - get time parameters (NTP daemon interface)
uintptr_t (3) - overview of system data types

--
Chris Green
·

Tristan Greaves <nospam.Tristan.Greaves@f1.n770.z30.fidonet.org> wrote:
> Re: How does Pi get its time?
> By: Chris Green to All on Thu Feb 10 2022 09:07 pm
>
> CG> How do Raspberry Pis know what the time is?
>
> CG> On my Pis NTP isn't installed but they seem to know what the time is
> CG> OK so how do they do it?
>
> If you are running Raspbian, it could well be timesyncd.
>
Ah, thank you. Why did they have to change the name from the almost
universal ntp I wonder.

--
Chris Green
·

On 2022-02-10, Chris Green <cl@isbd.net> wrote:
> Vincent Coen <nospam.Vincent.Coen@f1.n250.z2.fidonet.org> wrote:
> Well I can't see anything that looks like it:-
>
> chris@backup$ ps -ef | grep -i ntp
> chris 7885 7820 0 22:36 pts/0 00:00:00 grep --color=auto -i ntp
> chris@backup$ man -k ntp
> grantpt (3) - grant access to the slave pseudoterminal
> intptr_t (3) - overview of system data types
> mountpoint (1) - see if a directory or file is a mountpoint
> ntp_adjtime (3) - tune kernel clock
> ntp_gettime (3) - get time parameters (NTP daemon interface)
> ntp_gettimex (3) - get time parameters (NTP daemon interface)
> uintptr_t (3) - overview of system data types

You can look at the running systemd units by doing something like
`systemctl list-units` and see if systemd is running the time
syncing. If it's Raspbian, it's probably running through systemd.

On 10-02-2022 22:51, NY wrote:
> The Pi seems to have a crude
> real-time clock which gets the time approximately right (maybe wrong by
> a couple of hours), rather than reverting to "the beginning of time" (1
> Jan 1970).

It has no hardware clock; in a controlled shutdown or reboot it saves
the last known time to disk and reads that when it starts up again,
starts counting from there. So it will be behind by how ever long it was
down; maybe 15-20 seconds for a reboot? The same happens in the case of
sudden power failure, but I'm not sure how often it saves the current
datetime value during normal operation, so that may be why in that case
it is behind by more.

On 10-02-2022 22:34, Jim Jackson wrote:
> systemctl status systemd-timesyncd
>
> might need a sudo in front.

No sudo needed. Strange output on my Pi: says it has been running since
~2 minutes ago (that's about when I logged in) but this Pi is always on
and 'uptime' says I last rebooted 2 days ago.

On a sunny day (Thu, 10 Feb 2022 21:07:55 +0000) it happened Chris Green
<cl@isbd.net> wrote in <bebgdi-6i793.ln1@esprimo.zbmc.eu>:

>How do Raspberry Pis know what the time is?
>
>On my Pis NTP isn't installed but they seem to know what the time is
>OK so how do they do it?

On one of my raspberries I have a GPS module connected to /dev/ttyAMA0
it gets location and time even if there is no net connection.

On 10/02/2022 21:07, Chris Green wrote:
> How do Raspberry Pis know what the time is?
>
> On my Pis NTP isn't installed but they seem to know what the time is
> OK so how do they do it?
>

On my Raspberry Pi cards I always install NTP. As you say, sadly this isn't
the default on recent OS versions, so an extra step is required.

As often with Linux, the approach /appears/ to be "If it works, change it!".

--
Cheers,
David
Web: http://www.satsignal.eu

On 10/02/2022 21:51, NY wrote:
> "Knute Johnson" <knute2020@585ranch.com> wrote in message
> news:su409i$st3$1@dont-email.me...
>> On 2/10/22 15:07, Chris Green wrote:
>>> How do Raspberry Pis know what the time is?
>>>
>>> On my Pis NTP isn't installed but they seem to know what the time is
>>> OK so how do they do it?
>>>
>>
>> systemd.timesync.d
>
> Beware of running time-sensitive software on a Pi that reboots and doesn't
> immediately have an internet connection.
>
> I use a Pi to run Cumulus weather station software. This logs and graphs
> data from a weather station. If there has been a power cut, everything
> *starts* to boot at the same time when the power comes back. If the Pi boots
> first, and the router takes its time connecting, the Pi will run with a
> silly time until it is able to synchronise with an internet time source.
> This leads to graphs which suddenly shoot back by several hours (when the Pi
> reboots) and then shoot forward again (once the Pi has managed to
> synchronise). My impression is that the Pi checks frequently for a time
> source to begin with, which is why I rarely see the problem, but then starts
> polling less frequently, which is why on the rare occasions that I've seen
> the problem, the update to the correct time happens a fair time after the
> router has connected and I've been able to browse the web on other
> computers. The Pi seems to have a crude real-time clock which gets the time
> approximately right (maybe wrong by a couple of hours), rather than
> reverting to "the beginning of time" (1 Jan 1970).
>
>
> It's not the end of the world, and not worth the hassle of implementing a
> proper battery-backed RTC (as on a Windows/Linux PC). The software logs
> readings every 10 minutes (though it graphs them every minute) so there
> aren't usually many readings that have an incorrect timestamp, and it's
> obvious which times are missing because they *should* be at 00, 10, 20, 30,
> 40, 50 minutes past every hour. It's just a case of hand-editing the log
> file and then restarting the software manually so it regenerates the graphs
> with the correct time axis.
>
>
> I imagine that a Pi that is run without an internet connection (eg an
> unattended device with no Ethernet/wifi connection) will free-run - unless
> it has a GPS connection (eg to record a track of where it is going) in which
> case it can be configured to use the time from the GPS to sync the system
> clock. I imagine there are RTC modules that communicate through the GPIO
> socket, for devices that have no internet connection.
>

You can get a board with a GPS and RTC here:

https://store.uputronics.com/index.php?route=product/product&path=64&product_id=81

At least, when they become available again....

--
Cheers,
David
Web: http://www.satsignal.eu

On 11-02-2022 09:47, David Taylor wrote:
> On my Raspberry Pi cards I always install NTP.  As you say, sadly this
> isn't the default on recent OS versions, so an extra step is required.

And presumably you also uninstall the alternative systemd-timesyncd? I
*hate* the complexity of systemd vs the old sysvinit, but time sync is
one service that just works in the background, so I don't bother with it
& am not bothered by it. What are the specific advantages of ntp for you?

On 2022-02-11, A. Dumas wrote:

> On 11-02-2022 09:47, David Taylor wrote:
>> On my Raspberry Pi cards I always install NTP.  As you say, sadly this
>> isn't the default on recent OS versions, so an extra step is required.
>
> And presumably you also uninstall the alternative systemd-timesyncd? I
> *hate* the complexity of systemd vs the old sysvinit, but time sync is
> one service that just works in the background, so I don't bother with it
> & am not bothered by it. What are the specific advantages of ntp for you?

IIRC, installing the ntp package automatically removes the
systemd-timesyncd one.

I guess the main advantage of ntp is that people already know how to
configure it (if they want a custom configuration) whereas customizing
anything in systemd seems to involve a lot of learning and guesswork.

On Thu, 10 Feb 2022 18:11:47 -0500
Dennis Lee Bieber <wlfraed@ix.netcom.com> wrote:

> On Thu, 10 Feb 2022 22:38:16 +0000, Chris Green <cl@isbd.net> declaimed
> the following:
>
> >>
> >Ah, thank you. Why did they have to change the name from the almost
> >universal ntp I wonder.
>
> For the same reason the fairly easily understood Sys-V init
> processing is being replaced by the mysterious complicated systemd
> scheme... "It's new! Therefore it must be better!"

Sys V init is far from elegant (name ordered scripts ugh!) and
somewhat overengineered (run levels and all those links ugh!) so I can
understand the desire to replace it - but there was BSD init easily
available which is like Sys V init without the ugh! Then again I am sure
there are some who miss having everything in /etc/rc.

I have a slightly different theory to "It's new! Therefore it must
be better!" - I think it's more about separating Linux the OS family from
Unix the OS family - remember Gnu's NOT Unix.

--
Steve O'Hara-Smith
Odds and Ends at http://www.sohara.org/

On 11/02/2022 10:23, A. Dumas wrote:
> On 11-02-2022 09:47, David Taylor wrote:
>> On my Raspberry Pi cards I always install NTP.  As you say, sadly this
>> isn't the default on recent OS versions, so an extra step is required.
>
> And presumably you also uninstall the alternative systemd-timesyncd? I
> *hate* the complexity of systemd vs the old sysvinit, but time sync is
> one service that just works in the background, so I don't bother with it
> & am not bothered by it. What are the specific advantages of ntp for you?

No, the uninstall has not been an issue. The advantages are many, but briefly:

- able to accept a PPS time source (e.g. GPS)
- accepts multiple servers including "pool"
- acts as a time server for other clients
- simple setup
- easy to monitor (locally and remotely)
- having the same configuration and monitoring on Linux and Windows

--
Cheers,
David
Web: http://www.satsignal.eu

On 11/02/2022 11:56, David Taylor wrote:
> The advantages [of ntp] are many, but
> briefly:
>
> - able to accept a PPS time source (e.g. GPS)
> - accepts multiple servers including "pool"
> - acts as a time server for other clients
> - simple setup
> - easy to monitor (locally and remotely)
> - having the same configuration and monitoring on Linux and Windows

You forgot:-

- nothing to do with Lennart Poettering

As if you raised a bug report on timesyncd suddenly returning an extra
37 minutes in a day, he would close it with a "wont fix - correct on Mars".

---druck

On Thu, 10 Feb 2022 21:51:22 -0000, "NY" <me@privacy.invalid>
wrote:

> Beware of running time-sensitive software on a Pi that reboots and doesn't
> immediately have an internet connection.

Hint:
For services that require the correct clock (after initial
syncing with the internet) add the following to the
/etc/systemd/system/your.service definition of your servicve:

---
[Unit]
Description=your service
After=systemd-time-wait-sync.service
: --more here--
: [Install]
WantedBy=multi-user.target
---

and enable/start:

sudo systemctl enable systemd-time-wait-sync.service
sudo systemctl start systemd-time-wait-sync.service

--
Regards,
Kees Nuyt

On 10/02/2022 22:38, Chris Green wrote:
> Tristan Greaves <nospam.Tristan.Greaves@f1.n770.z30.fidonet.org> wrote:
>> Re: How does Pi get its time?
>> By: Chris Green to All on Thu Feb 10 2022 09:07 pm
>>
>> CG> How do Raspberry Pis know what the time is?
>>
>> CG> On my Pis NTP isn't installed but they seem to know what the time is
>> CG> OK so how do they do it?
>>
>> If you are running Raspbian, it could well be timesyncd.
>>
> Ah, thank you. Why did they have to change the name from the almost
> universal ntp I wonder.
>
Because Lord High Arsehole Poettering said so. He who Must Be Obeyed...

--
Any fool can believe in principles - and most of them do!

On 2022-02-11, Ahem A Rivet's Shot <steveo@eircom.net> wrote:

> On Thu, 10 Feb 2022 18:11:47 -0500
> Dennis Lee Bieber <wlfraed@ix.netcom.com> wrote:
>
>> On Thu, 10 Feb 2022 22:38:16 +0000, Chris Green <cl@isbd.net>
>> declaimed the following:
>>
>>> Ah, thank you. Why did they have to change the name from the almost
>>> universal ntp I wonder.
>>
>> For the same reason the fairly easily understood Sys-V init
>> processing is being replaced by the mysterious complicated systemd
>> scheme... "It's new! Therefore it must be better!"
>
> Sys V init is far from elegant (name ordered scripts ugh!) and
> somewhat overengineered (run levels and all those links ugh!) so I can
> understand the desire to replace it - but there was BSD init easily
> available which is like Sys V init without the ugh! Then again I am
> sure there are some who miss having everything in /etc/rc.
>
> I have a slightly different theory to "It's new! Therefore it
> must be better!" - I think it's more about separating Linux the OS
> family from Unix the OS family - remember Gnu's NOT Unix.

That reminds me of a sales pep rally that I wasn't allowed to duck
out of. The bigwigs were going on about "product differentiation" -
which I interpreted as: "Make it different, even it it means
screwing it up."

--
/~\ Charlie Gibbs | Microsoft is a dictatorship.
\ / <cgibbs@kltpzyxm.invalid> | Apple is a cult.
X I'm really at ac.dekanfrus | Linux is anarchy.
/ \ if you read it the right way. | Pick your poison.

On Fri, 11 Feb 2022 22:35:55 GMT
Charlie Gibbs <cgibbs@kltpzyxm.invalid> wrote:

>On 2022-02-11, Ahem A Rivet's Shot <steveo@eircom.net> wrote:
>
>> On Thu, 10 Feb 2022 18:11:47 -0500
>> Dennis Lee Bieber <wlfraed@ix.netcom.com> wrote:
>>
>>> On Thu, 10 Feb 2022 22:38:16 +0000, Chris Green <cl@isbd.net>
>>> declaimed the following:
>>>
>>>> Ah, thank you. Why did they have to change the name from the almost
>>>> universal ntp I wonder.
>>>
>>> For the same reason the fairly easily understood Sys-V init
>>> processing is being replaced by the mysterious complicated systemd
>>> scheme... "It's new! Therefore it must be better!"
>>
>> Sys V init is far from elegant (name ordered scripts ugh!) and
>> somewhat overengineered (run levels and all those links ugh!) so I can
>> understand the desire to replace it - but there was BSD init easily
>> available which is like Sys V init without the ugh! Then again I am
>> sure there are some who miss having everything in /etc/rc.
>>
>> I have a slightly different theory to "It's new! Therefore it
>> must be better!" - I think it's more about separating Linux the OS
>> family from Unix the OS family - remember Gnu's NOT Unix.
>
>That reminds me of a sales pep rally that I wasn't allowed to duck
>out of. The bigwigs were going on about "product differentiation" -
>which I interpreted as: "Make it different, even it it means
>screwing it up."

Hence disastrously obscure and degraded user interfaces :(

--
Basic