I have a thought. The quote isn't "since Hastings" it's "since BEFORE Hastings". What if (as usual) she is feeling the literal truth, giving a point of reference Harry would understand, and hiding the actual truth all at the same time. She does mean 1066, but not October, what if the actual event was marked by Halley's comet? Maybe they had a right of Ascension then and the invasion and Norman Conquest was a side effect of the new balance.

 If we assume an average of one year passing in each book and 7 books left till the BOAT, that gives us a new moon on the night of Oct 31/Nov 1 2024. That would be a perfect start the Empty Night, right.