Running 10.8 Mountain Lion, I'm trying to create a new event with Applescript like this:

set theDate to (current date)
tell application "Calendar"
tell calendar "Calendar"        
    set timeString to time string of theDate
    set newEvent to make new event at end with properties {description:"Last Backup", summary:"Last Backup " & timeString, location:"To a local unix system", start date:theDate, end date:theDate + 15 * minutes, allday event:false, status:confirmed}
    tell newEvent
        delete every display alarm
        delete every sound alarm
        delete every mail alarm
        delete every open file alarm
    end tell        
end tell
end tell

However, this does not remove the default Calendar alert which one can set through Calendar preferences (30 minutes prior in my case).

How do I create an event with no alarms at all through Applescript?