It looks like I can’t do it. In settings of this widget (systray) I can only switch on/off elements but not rearrange like I want.
I want it to be like this (in Manjato XFCE I can do it easy)
окт
27 пт
12:30
It’s in Russian, so in English it will be…
oct
27 fri
12:30
In short, you cannot. The order of the items in the Plasma system tray used to be determined by their starting order, but this proved too random, and a few Plasma generations ago, it was decided to order them alphabetically, albeit that with some applications — e.g. strawberry — this isn’t consistent.
Edit the format. There should be a help text on account of what variable substitutes what value. 
No, editing format won’t help.
I can switch on/off a date and configure it with format (MMM d ddd). But it always FOLLOW time, It can’t be in front of it.
The options are
Ok. Could you show me format string you use to configure clock widget?
I think it depends on your locale settings. I see no way to set it in the clock itself anymore — it used to be there, but now it’s not.
I guess it depends on your LC_TIME, which in my case is set to en_GB.UTF-8.
What’s wrong? Your settings are so special that I can’t reproduce or what?
Which part of… 
… is it that you do not understand? 
My LC_TIME is ru_RU.UTF-8
So why can’t I put date in front of a time?
I don’t know. It used to be possible to do this, but they changed the settings somewhere along the line, and normally the date should be in front of the time.
One thing you can try is right-click the panel and choose “Enter Edit Mode”, and then “Show Alternatives”. It used to be possible to substitute the clock for the event calendar, which used to allow — at least, again in the past — a custom format.
Also, in my clock settings, I have the date set to “long format”. Maybe that makes a difference?
Another option would be to install the Event Calendar widget and use that instead of the default one. Here’s mine:
… Which is what I said already…
It either way seems to depend on the locale settings. 
It is quite possible to create a customized locale file.
The locale layout is defined in relevant locale file.
The locale file can import other files - so in your case you could create a copy with a slightly different name - then use that - do not modify the system file - it will be overwritten without any warning.
I have no experience with this - read the above linked topic - read the article Locale - ArchWiki
Copy the file to your home
cp /usr/share/i18n/locales/ru_RU ~/my_RU
Then modify - possibly use the en_DK as template or guidance, save the file and copy back
sudo cp ~/my_RU /usr/share/i18n/locales
Make the necessary changes to /etc/locale.gen (run locale-gen) , locale.conf and ~/.config/plasma-localerc accordingly.
There is absolutely no guarantee that you will be able to achieve the desired result - but - boy - you are in for some learning …
The Plasma 5 HTML Clock should do the job. It can be added to panel & desktop. Some knowledge of HTML may be required.
Perhaps this helps:
Not to be a party pooper, but that article is from 2021.
Plasma no longer randomly orders the icons in the system tray, and hasn’t been doing that anymore for about a year, if not longer. The order is now alphabetic. 
Noticed too late, and yeah. But I DID specify that I don’t know…
You can change the Date and Time format by editing the file
/usr/share/plasma/plasmoids/org.kde.plasma.digitalclock/contents/ui/DigitalClock.qml
Take a look at ‘Event Calendar’ widget - I use that for my panel clock, and you can pretty much edit that any way you like (including specifying colour).
You can define 2 lines display…
‘‘ddd dd MMM’’ for the date, I only specified ONE line, I entered two widgets because I use the date to access my schedule. The other widget just displays the time HH:mm with Atkinson Hyperlegible font. You can swap them if you like
However, I tend to have that panel hidden accessed only for calendar entry/scheduling. The best onscreen time/date is by far via a couple of conkies:
The date stays on the desktop, but the time stays on top so I can see it when I watch TV on Plex maximised (won’t work fullscreen, so I do maximise and no border).
conky.config = {
– Conky settings
background = true,
update_interval = 300,
double_buffer = true,
no_buffers = true,
– Window specifications
gap_x = 240,
gap_y = 55,
alignment = ‘top_right’,
own_window = true,
own_window_type = ‘dock’,
own_window_transparent = true,
own_window_argb_visual = true,
own_window_argb_value = 50,
own_window_hints = ‘undecorated,below,sticky,skip_taskbar,skip_pager’,
own_window_class = ‘conky’,
– Text
use_xft = true,
xftalpha = 0.1,
font = ‘Arial Rounded MT bold :size=14’,
default_color = '#646464',
};
conky.text = [[${time %A %d %B %Y}
${font :size=10}${utime %H:%M %Z} ${goto 105}${tztime America/Chicago %H:%M %Z} US
]];
conky.config = {
– Conky settings
background = true,
update_interval = 5,
double_buffer = true,
no_buffers = true,
– Window specifications
gap_x = -670,
gap_y = 5,
alignment = ‘top_middle’,
own_window = true,
own_window_type = ‘dock’,
own_window_transparent = true,
own_window_argb_visual = true,
own_window_argb_value = 50,
own_window_hints = ‘undecorated,above,sticky,skip_taskbar,skip_pager’,
own_window_class = ‘conky’,
– Text
use_xft = true,
xftalpha = 0.1,
font = ‘Arial Rounded MT bold :size=24’,
default_color = '#646464',
};
conky.text = [[${time %H:%M}]];