In Skyrim, I was reading a book which described a date in the First Era. It's some kind of research note (can't remember where its from) and when dating an event he says something along the lines of "1E2123 (4243 in the old calendar)". It's possible that when the Aylieds started their own independent state, they thought they were so good that they started their own calendar - The era of the almighty Aylieds or something similar. When the Allesian rebelliion happened, they decided to keep the calendar, but rename it the first era - as in the first era of men (after the merethic era).
That rings a bell - I think I've read the same note. You might be on to something... Maybe originally it was 'The Merethic Era' and 'The First Era of Men' and the second, third, fourth eras followed on naturally from that.
The dates on the book would be evidence that they did, unless for some reason it were a forgery, which doesn't make sense given the context. Writer's mistake or not, it's lore now!
Fair point. Just like 'Elsweyr' is lore, even though I'm sure modern Bethesda writers wish they could get rid of the bad pun in their serious fantasy setting