You are correct: Millennia (with two n’s) has been the traditional spelling. However, as with so many other words, what was once a misspelling has now become acceptable. Thus, “Millenia” now passes muster. I used to be a stickler for traditional spellings and usages, but it got to the point I was fighting a losing battle. It’s sad, really. We have become lazy, and are now accepting things we never would have even considered a couple generations ago. I’m glad you retain the traditional approach: We need more of it.
Thanks, It's often a lonely line to hold but I'm growing into the role of kindly mossback. (It's with a bit of a shock that I recently noticed that on my technical committees I'm often the oldest one there; and I have no plans for retirement soon.)
English spelling is anything but logical but I'm holding to what I've learned, at least until we get new rules. By comparison Gaelic looks complex but is extremely phonetic; learn a few simple rules and you have it. I'd like to see that, but can't get past the mainland vs Taiwan Chinese difference: the communists "simplified" the characters,and it's not clear to me how much either side can read the other's writing anymore.
There is a LOT to be said for tradition, but few are saying it.