I think the key word in the directions is exclusive so whatever time you specify will not be included in the date range.
So in your case "2014-12-21T00:00:00.000Z" would mean that the event would no longer exists at the very beginning of 12-21. If you want the event to go through 12-21 you’d want to set the end time to "2014-12-22T00:00:00.000" (first possible time in 12-22).