It’s taken me the longest time to figure this out: an AppleScript to set the date on a Post in MarsEdit.

tell application "MarsEdit"
	make new document
	 tell front document
		set published date to date "Sunday, 4 June 2023 at 09:01:02"
	end tell
end tell
Screenshot of the script with a highlight that the word date just before the actual date is crucial.