This Time Stamp Automator Service (Dropbox download) will place the current date-time in a cell when you hit a keyboard shortcut.
To install double-click the downloaded .workflow package and if necessary give permission at System Preferences > Security & Privacy. Also make sure Automator.app and Numbers.app are listed and checked at System Preferences > Security & Privacy > Privacy > Accessibility.
Time Stamp should now be listed in your Numbers > Services menu.
To attach a keyboard shortcut go to System Preferences > Keyboard > Shortcuts > Services. Scroll down in the right panel until you see Time Stamp. Double-click on the 'none' next to it and enter your shortcut (shift-command-T works well on my machine).
This installation and setup should take a minute or so.
Thereafter, to put a timestamp in a Numbers cell all you have to do is place the cursor in the cell and type shift-command-T (or whatever shortcut works for you). The result will appear however you have formatted that cell. You can display Fri, 18 Jan 2019 3:37 pm or whatever format you want that is valid in your region. You choose the format from the dropdown in the Data Format section of the Cell tab in Format panel at the right.
This may be a "workaround" but it takes fewer keystrokes than other workarounds (such as copying and pasting from a cell with NOW) and is just as efficient as the built-in ctrl-shift-; timestamp shortcut in Excel.
The simple AppleScript within the Automator Service can be viewed by holding down the option key in Finder and (in the menu) Go > Library > Services and double-clicking Time Stamp.workflow. The script is repeated below.
SG
on run {input, parameters}
set cd to (current date)
set the clipboard to cd's date string & " " & cd's time string
tell application "Numbers" to activate
tell application "System Events"
keystroke "v" using {option down, shift down, command down}
end tell
end run