In the earlier editions, the Order of Hermes used the "Age of Aries" calendar instead of the christian calendar. However, according to the book Mythic Locations, in the chapter "The Fallen Temple of Mercury", Mythic Europe is in the historically acurate Age of Pisce.
So, what exactly does the "AA" calendar means?
Magi still use the Age of Aries, even though it's been so long that the world is in the Age of Pisces. The Order likes to use Roman-sounding things to claim descent from Roman magi, hence all the magi with Latin names and all the other archaisms and anachronisms that magi seem to like.
OoC it was an error in earlier editions, and some of the authors (not me this time) decided to use it as a plot hook rather than correct it.
The Age of Aries started sometime around 2000BC and ended around 1AD. Exact start dates vary by source, over a period of about 150 years in ether direction. The end date is almost always at or around the same from most sources though. It is generally considered to mark the change from polytheism to monotheism.
So the Order never existed during the Age of Aries. They have existed entirely within the Age of Pisces. Currently we are at the end of the Age of Pisces or the beginning of the Age of Aquarius depending on which varies methods you use to determine the start date of an Age (or even how long they last. Some methods use 2,000 years, while others use 2,150 years).