How can I change this system tray ugly date visual?

I don't have that box, not even when I hover my cursor over the entry - but I do make the dialog appear by clicking on the time in the SysTray/Notn area not by using the keyboard..
What system changes were made shortly before the box started appearing?
Does the problem occur in Safe mode?
Does the problem occur for another user account?

[Changes made after seeing Bree's excellent response below].

All the best,
Denis

Paultx said:

I don't like this ugly rectangle around the date. Besides, I think it's unnecessary. Is there a setting I can use to remove it? TIA.

Try3 said:

I don't have that box, not even when I hover my cursor over the entry.

You get that box if you open the time/date using the keyboard shortcut WinKey+Alt+D. Because you have indicated that you want to use it from the keyboard the box is there to show which item currently has focus. You can move the focus to the next item with the Tab key and, when a date number in the calendar has focus, the arrow keys.

Each selectable item has a different function, invoked by pressing the space bar or Enter (or by clicking on it). The function of the date at the top is to reset the currently selected day in the calendar to today's date.

You should not get that box if you click on the date in the taskbar to get it to pop up. But if you then want to use it from the keyboard you can make the box appear by pressing the Tab key.

You should not get that box if you click on the date in the taskbar to get it to pop up.

- in fact I find I do...

..but if you notice, that's a very old article....

That's certainly not obvious!