Shell – way to adjust the formatting of the date/time in the GNOME 3 login screen and desktop

The ArchWiki is a very good source of information. This is where I found the following:

To configure the GDM theme use this command:

sudo -u gdm dbus-launch gnome-appearance-properties

For more configuration options, use this command:

sudo -u gdm dbus-launch gconf-editor

And modify the following hierarchies:

/apps/gdm/simple-greeter
/desktop/gnome/interface
/desktop/gnome/background

You may end up with an Xauth error. If that happens, try gksudo instead of sudo. If the error persists, you can do this:

xauth list $DISPLAY

This gives you the xauth cookie being used by your user. Copy the output, then run the following, replacing "" with the output of the previous command.

sudo su -u gdm
xauth add <cookie>
dbus-launch gnome-appearance-properties

This logs you in as the gdm user, adds the cookie, permitting gdm to use your display, and launches gnome-appearance-properties.

It took me half a day to (almost) fully investigate this, but I think I found how to do it, and you’re not gonna like it …

The Solution:

In a nutshell,

KDE just uses QT’s QLocale. Which itself uses hard-coded data inside qlocale_data_p.h, deep in QT’s core library code.

This data is apparently manually generated from the Unicode Consortium’s “Common Locale Data Repository”, using several Python scripts inside util/local_database/ of the QT source code package. Which itself is not even part of the source code, I am certain. Let alone, using any kind of configuration or data files on your computer.

So the only way to do this, is to …

1. Download the CLDR files, either as a package, at http://unicode.org/Public/cldr/latest/core.zip (or cldr-common-*.zip, they both seem to contain the same), or at https://www.unicode.org/repos/cldr/trunk/, and extract the XML files under common/main/.
   Gentoo also has a package called app-i18n/unicode-cldr btw.
2. Acquire the QT core source code, e.g. from your distribution’s repository, and unpack it.
3. Use the scripts in util/local_database/ (like cldr2qlocalexml.py and qlocalexml2cpp.py), with your language of choice’s XML file, to re-generate the static qlocale_data_p.h.
4. Then compile and install the package as normal.

And don’t forget to recompile all other packages that include qlocale_data_p.h.

Or, on Gentoo, make a lasting patch with:

ebuild $(equery w dev-qt/qtcore:5) prepare
pushd /var/tmp/portage/dev-qt/qtcore-5*/work/ # assuming default Portage build directory
mv qtbase-opensource-src-5.9.3 a
cp -a a b

NOW make the above changes in b, and not in a.

mkdir -p /etc/portage/patches/dev-qt/$(basename $(dirname $(pwd))) # only for this version
#mkdir -p /etc/portage/patches/dev-qt/qtcore # for all versions
diff -ur a b > /etc/portage/patches/dev-qt/qtcore*/my-locale.patch
rm -rf a b
popd
ebuild $(equery w dev-qt/qtcore:5) clean
# Rebuild all packages that depend on it, just to be sure. May be optional.
emerge -1 dev-qt/qtcore:5 $(equery d dev-qt/qtcore:5 | sed 's/^/=/')

Final thoughts

To be perfectly frank, I didn’t check if those Python scripts actually worked by just passing an XML file. As, at that point, I just stopped bothering, and wanted to kill it with fire. :)

If anyone wants to actually do it in practice, please report, and I will update this answer. (Or do it yourself, if you can)

Here’s how I got there:

• I opened KDE’s time formats setting dialog, via right-click on the task bar clock.
• Then, with htop I found the most recently started processes, which included the obvious kcmshell5 formats.
• Using htop’s list open files feature on it, I filtered for “format”, and found /usr/lib64/qt5/plugins/kcm_formats.so.
• With the help of Gentoo’s equery belongs /usr/lib64/qt5/plugins/kcm_formats.so I could identify the package kde-plasma/plasma-desktop.
• After unpacking its source file, I quickly found kcms/formats/kcmformats.cpp, whose addLocaleToCombo used the QLocale, and included <QLocale> too.
• This QLocale could then be found with locate -i QLocale to reside at /usr/include/qt[5]/QtCore/qlocale.h. It included some generated enumerations of the languages, scripts, etc.
• Another equery belongs /usr/include/qt[5]/QtCore/qlocale.h pointing me to dev-qt/qtcore/qtcore, and unpacking the source file later…
• … I already had a gut feeling, that it was all generated with some tool. So I looked into util/ and found util/local_database/README stating “local_database is used to generate qlocale data from the Common Locale Data Repository (The database for localized names (like date formats, country names etc)).”.
• Having never heard of that, I looked it up, and it’s apparently by the Unicode Consortium. The code of cldr2qlocalexml.py was not very useful though, as it is called with the path to a directory containing the CLDR locales. Its result is used for qlocalexml2cpp.py though, which generates the file src/corelib/tools/qlocale_data_p.h, which includes huge hard-coded tables of all the locale data. And that file already existed in the source. So… yep, it’s (partially?) hard-coded.
  
• But the CLDR XML files were nowhere to be found. So I assume it just uses the previously prepared qlocale_data_p.h. Which is not very in the spirit of open source. But at least you can do it yourself, as described above.