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

Excuse my ignorance but how would you execute this script?

Copy/paste the script into the "Script Editor" application (Applications -> Utilities), press play.

Or you could save this to a file, "Purge Old Reminders.scpt," and then from Terminal use the command line program osascript,

osascript "Purge Old Reminders.scpt"

You'll be asked to grant Terminal permission to access your Reminders.

Any way to execute this from a Windows PC or directly from iPhone?

There are ways to trigger AppleScripts from an iPhone, but you definitely have to have a Mac in order to run the AppleScript.

Is this script permanently activated upon pressing "play" button in script editor? If so, how do you disable it afterwards? @drdrang

Like all AppleScripts, it runs until it's finished or encounters an error. But there's nothing in the script that causes it to run repeatedly. If it is running for a long time for you, chances are you have a huge number of completed reminders. I've learned since writing this script that AppleScript sometimes takes a ridiculously long time to process the every reminder whose clause. If that's what your question is about, you can stop the process in Script Editor by pressing the Stop button (black square) near the upper left corner.

If anyone has issues with this script try changing 30 to 1 in the second line so that is deletes the tasks no matter how long ago you deleted them.