How to display date under time in GNOME

Use clock override extension to achieve this.

After opening the extension's preferences, under "text to display instead of the clock" tab enter %I:%M %p%n %d/%m/%Y.

%n is responsible for next line and for the rest of formats check here.

this answer is only for "text alignment" purpose in continuation with the comments below the question and the already accepted answer.

when we enter next line by %n in the below field, the lines are aligned to the left as shown in the below image.

to make this alignment to the center, we need to open the gdm3.css file and edit the portion like below.

open the file by sudo -H gedit /etc/alternatives/gdm3.css find the below section under "TOP BAR" line number around 743.

#panel.solid .panel-button:checked {
        color: white; }
    #panel.solid .system-status-icon,
    #panel.solid .app-menu-icon > StIcon,
    #panel.solid .popup-menu-arrow {
      icon-shadow: none; }

and add the below lines under the line icon-shadow: none; }

#panel .clock-display {
   text-align: center;
}

it looks like below after adding the required lines.

#panel.solid .panel-button:checked {
        color: white; }
    #panel.solid .system-status-icon,
    #panel.solid .app-menu-icon > StIcon,
    #panel.solid .popup-menu-arrow {
      icon-shadow: none; }
#panel .clock-display {
   text-align: center;
}

Save the file & Close.
Reboot to see the change.

Note: if you use different shell themes than the default shell theme, you may search for the text clock-display in the respected gnome-shell.css file. if found, edit there, if not found add the lines as shown above.