How to change system time format to specific standard?

I don’t want my date-time representation to change per the country I’m in; I want to be able to choose between specific standards, such as RFC 3339 or ISO 8601, the latter being my preference.

I am easily able to do so on Windows, but don’t appear to be able to do so in the regional settings KCM. Is this possible?

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If you want to use the ISO 8601 standard use en_DK.UTF-8 in the Time category in Region & Language KCM.

See 3.3 in the Arch Wiki Locale - ArchWiki

Seems to work fine in the terminal but the Plasma tray Clock has some issues with it?

€: corrected to en_DK



Typing en_US.UTF-8 into the search bar doesn’t work, so I tried en_US

en_US shows the correct date format in its preview here

However, it then shows the standard US preview when selected

and then doesn’t work.

PS /home/rokejulianlockhart> # en_GB.UTF-8
PS /home/rokejulianlockhart> who -b  
         system boot  2023-05-09 17:08
PS /home/rokejulianlockhart> # en_US.UTF-8
PS /home/rokejulianlockhart> who -b
         system boot  2023-05-09 17:08

so I think who -b is perhaps unaffected…?

That’s controlled separately, unfortunately

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Sorry I posted the wrong one, I meant en_DK.


That also uses {day}/{month}/{year} like en-GB does.

Hey @rokejulianlockhart,

after setting it to en_DK.UTF8 it looks like this for me:

after setting it to de_DE.UTF8

You are correct that the ẁho -b command gives the same output.
The date command gives a different output.

Also the System Clock changes. Although not in the way I expected it.

I’m a little bit confused now :smiley:

PS: I uncommented the relevant locales in /etc/locale.conf and run locale-gen (commands might not apply to your Distri.)

Also restart was required.

€: en_DK.UTF-8 also gives me the same preview, which seems incorrect see first screenshot.

PS /home/rokejulianlockhart> who -b
         system boot  2023-05-21 18:09
PS /home/rokejulianlockhart> date          
Sun 21 May 22:33:47 BST 2023


Thanks for your assistance, @Duha – I’ll probably just file an FR at rather than potentially corrupt configuration files, since that wouldn’t do any less technically competent users any good (and Plasma should have at least feature parity with Windows).

I’ve changed the topic type from #help to #brainstorm because this is obviously currently impossible. provides a decent demonstration of how to easily design the GUI selector:


Sure, and that works fine in a single app. But do you really want to re-define your favorite date format in every single app? Probably not. Which is why it needs to be done centrally, in one place, in a way that all apps that ask for date formats can consume. Which is a lot harder, because the POSIX data standard currently does not easily support defining arbitrary date formats.

In KDE 4 times, we let you define arbitrary custom date formats, bypassing the POSIX locale standard, but it only worked for KDE apps that specifically knew how to read the KDE-defined date format. So people complained that GNOME and other apps didn’t get the same date formatting, and there was no way to fix this issue in principle because we were explicitly ignoring the cross-desktop standard and the GNOME folks (to say nothing of all other software) weren’t going to adopt the KDE thing as a new de-facto cross-desktop standard to replace the existing one. So, in Plasma 5 times, we admitted defeat and went back to supporting POSIX locales so now the format you choose applies to all apps. But as a consequence, you can’t arbitrarily customize the date format.

Those are the limitations and external conditions involved.

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Who could create a more modern standard? Systemd?

It’s far underneath them; it would be the IEEE Computer Society, which maintains POSIX standards.

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Idk, maybe its just me, but I was hoping for a more modern standard and using posix just as a fallback if the more modern standard is not available.

Posix: Latest version IEEE Std 1003.1-2017
2017; 6 years ago

(Most) modern Linux Distris already embrace modern technology like systemd, wayland etc…, who knows what the BSD’s and a few small Distris will do? Maybe I’m just naive, but I wish we would move on from projects that want to stay in 1980.

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Yeah, that was always my suggestion, @ngraham. I don’t believe I’ve stated otherwise, but thanks.

I wholeheartedly agree, @Duha. I believe that

was a retrograde step, as I’m sure the developers who had to choose this path thought too. I don’t mind having to set two similar preferences – we already have the ability to choose a different GTK theme preference. Where it’s necessary, I think that the “Powerful when needed” part of the KDE mantra applies here.

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