Do any of the books give any information on the calendar the Order uses?
I have found references to the "Age of Aries" in various books, though a cursory google search indicates that this Age ended over a thousand years before the game is set. Does the Order use an outdated calendar, or something more obscure?
Do the months of the year follow the modern zodiac? With similar starting and ending times during the year? Which month is first in a year?
I assume it's an astrological calendar of some sort dealing with when a particular celestial object is in that particular constellation. I don't know that I've seen it clarified for ArM5 yet.
As far as the year goes, we use the solstices and equinoxes to identify when the year starts, as they're nice and easily identified... mind you, this does cause a little overlap as winter of say 1215 goes into 1216, but we're ok with that. (Winter is mid-Dec through mid-March, Spring mid-March to mid-June, etc...)
-Ben.
The ages of Aries are used in 4th Ed quite extensively. Bonisagus swore his oath in the 950th year of Aries (767 AD).
Regretably I can't find a good writeup of it anywhere.
Ah, the question of the ages.
It is best dealt with The Mysteries (the 4e book, not The Mysteries Revised Edition, which is for 5e) on page 59-60. Essentially, the Order uses the standard astrological calendar (you can read all about it on the interweb), with each Season starting at the appropriate solstice/equinox, but has the age "wrong". TM presents several options for the SG:
It's the Age of Pisces: Just use the real-world age.
You're Not in Kansas Anymore: In ArM, it's the Age of Aries.
Ancient Sympathies: It's part of preserving the ancient magics of the bygone age of magic.
Turning Back Time: It's part of a secret conspiracy to turn back the clock of the ages and return the world to the time of magical greatness.
The TM description is somewhat more extensive, and in general the astrology chapter there is excellent if you care for such stuff. But that's the gist of things.
Meanwhile, in the real-world, I suspect the reason for the inconsitency is that some ancient author got his ages wrong.