How do I customize the date/time display in GNOME Shell's top bar?

You may achieve your goal using some GNOME shell extensions.

Moving clock to the right

You may use Frippery Move Clock.

See this Q&A for more options.

Changing date & time format

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.

I used

dconf write /org/gnome/shell/extensions/panel-date-format/format "'%a (KW %V) %d.%m.%y %H:%M'"

to display below format:

Another place to look:- Ubuntu Software Centre - certainly in Ubuntu 17.10. If you search for "clock" a whole list of different programs is displayed, Clock Override among them.