AppleScript to delete all reminders more than 30 days old.

Purge Old Reminders.scpt

set remindersOpen to application "Reminders" is running
set monthAgo to (current date) - (30 * days)

tell application "Reminders"
	set myLists to name of every list
	repeat with thisList in myLists
		tell list thisList
			delete (every reminder whose completion date is less than monthAgo)
		end tell
	end repeat
	if not remindersOpen then quit
end tell