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?