Here's the whole thing, shamelessly stolen directly from his blog (where he's responding to this post I wrote around Christmas):
So bls was asking for information on the antiphon “Hodie Christus Natus Est” used with the Magnificat on the Second Vespers of Christmas in the Roman liturgy as it appears in the Liber.
She was wondering about its antiquity. This piqued my interest, of course, so I thought I’d hop over to my favorite collection and check it out… Consulting the Winter volume of the Hartker Antiphonary (Cod. Sang. 390) I went to the folio for the Gospel Antiphon at Vespers and found the following:
As you’ll recall, scribes weren’t always picky about their line breaks… The “In Ev” in the upper right lets us know that the next line will be a piece used with the Gospel (Canticle); the red A in the left margin tells us that this line is an antiphon. Clearly this is not the antiphon we’re looking for, though.
Paging back, however, I ran across this:
Here we have the end of Lauds. The first line in this clip is the Gospel Canticle on the Benedictus which is the same as what’s in the Liber. That’s the end of Lauds proper—then we find our missing antiphon!
What we get are two antiphons here before we arrived at the antiphons used for the Little Hours during the day, the start of which is signaled by the “Ad Cursus” (For the Round of the Day) rubric at the bottom of the clip. These two antiphons are marked “Ad Crucem” which lead me to believe that it’s being used as part of the Common Commemoration to the Holy Cross. Typically these involve the use of an antiphon, a versicle and response and a concluding collect. This book tends to have them following Lauds and occasionally after Vespers. I’m not clear why there are two here, however…
Interestingly the Portiforium of St Wulstan from around the same time shares the Lauds antiphon but has a different Vespers antiphon (the “Lux orta” that the San Gall appoints for Prime, actually). The “Hodie Christus natus est” doesn’t appear in that book at all. Nor does it have antiphons for commemorations.
All of the English monastic breviaries studied at the Cursus Project use Hodie Christus natus est for the 2nd Vespers of Christmas with one fascinating exception. The Worcester Antiphoner uses the same 2nd Vespers Antiphon that we saw in the San Gall manuscript and the Hodie Christus natus est doesn’t appear until St Johns day where it is used as a Commemoration of the Nativity (Again, antiphon, versicle & response, then a collect—this time the one from Christmas).
The Sarum Breviary, following in the footsteps of the others uses the now standard Hodie Christus natus est.
So, that at least fills in a few points in regard to the history of this particular antiphon and its circulation in Northern Europe and England.
And this did answer my question; this manuscript comes from 990-1000 - which means that this antiphon is at least a thousand years old.