customize top panel clock in Ubuntu 18.04 [duplicate]
Solution 1:
Once you installed the gnome-shell extension clock-override
Open the extension preferences and change the values as shown below..
When you click the link "What do all these %x codes mean?" at the bottom right side of Extension preferences dialogue..
this link GNOME Developer site opens, from which you can understand and adjust the format/formats according to your wish..
%F: equivalent to %Y-%m-%d (the ISO 8601 date format)
%A: the full weekday name according to the current locale
%T: the time in 24-hour notation with seconds (%H:%M:%S)
Another Customization example: