Why does the Empire of Mankind still refer to dates based on the Common Era calendar?

Question

In Earths history, we've had quite a few calendars, each one marking its own "Year 1" - e.g., the founding of Rome; birth of Jesus; Mohammed's emigration to Medina; the French revolution; etc... In other words, when something REALLY important happens people tend to say "Let's reset the calendar!"

Why then, was such important event as Emperor's Ascension not marked as such? The empire still uses a calendar based on the ancient Earth one.

Out-of-universe, the answer is obvious: Warhammer 40,000 sounds much more futuristic than "Warhammer 10,000 (After Ascension? After Golden Throne?).

Is there in-universe reason for keeping the old calendar?

Answer 1

According to the Warhammer 40K wikia, itself sourced from the 40K rulebook (I don't have a copy any more so can't verify) in the 41st millenium the reason for the year numbering (i.e. the A.D. system from the Gregorian calendar) is no longer known, having been lost in the mists of antiquity and aeons of war and strife.

However the long history of humanity and the Imperium is now grouped into "Ages" (e.g. Age of Terra, Age of Technology). The current in-universe period is known as the "Age of the Imperium", which is marked from when the Emperor ascends to his throne and unifies humanity in the 31st millenium.

So although I can't find an explicit in-universe reason for keeping the same calendar I think it's implied that Gregorian calendar must have survived for at least 30,000 years before the time of the Emperor, by which time it would have become used and embedded on a galactic scale and changing it would have been extremely difficult. The events of major significance are now marked as the start of a new "age".

Answer 2

There is a difference between the Gregorian calendar and the AD dating it uses. AD dating was first proposed in the sixth century AD and was gradually adopted by various societies, being adopted by Portugal, the last holdout in Western Europe, in the 14th Century AD, two centuries before the Julian Calendar was modified to make the Gregorian Calendar.

The first major power to use AD dating in documents was the Holy Roman Empire. So if the first Warhammer Emperor claimed to be the heir or successor of the Holy roman Emperors it would make perfect sense for him to keep using the year count they used instead of decreeing a new year one. But since he probably doesn't claim that I have no idea why he kept the old year count which made it seem like the foundation of his empire was no big deal.