Time indicator is shown as
"%a %H:%M" in the center of the top bar.
How do I move it back to the right where it belongs, and make it include the full date and time (
"%Y-%m-%d %s %H:%M")?
You may achieve your goal using some GNOME shell extensions.
You may use Frippery Move Clock.
See this Q&A for more options.
You may use Clock override. It lets you
Override the Gnome Shell clock with a new time format or text of your choice.
It supports Python's
strftime format (so you'll be able to set your preferred
"%Y-%m-%d %s %H:%M" format to display).
Panel Date Format is another GNOME Shell extension that allows you to modify the formatting of date and time in the top panel, e.g. to include the current week number.
dconf write /org/gnome/shell/extensions/panel-date-format/format "'%a (KW %V) %d.%m.%y %H:%M'"
to display below format: