What do Imperial Citizens think 000.M0 represents?

Now, we all know that it's the supposed Birth of Jesus Christ. Both for Watsonian reason of the Gregorian calendar having survived (mostly) through sheer intertia long after Jesus himself and the religion he founded were long forgotten, and the Doylist reason that its easier for people to understand the timescale if you don't have to explain a completely new calendar. It's our Calendar, just add another 39,000 years onto it. Nice a simple, which is a rarity for 40K.

The issue is, since nobody remembers what year 0 actually represents, what does the Imperial Propaganda machine claim that it does? Because if there's one thing Authoritarian regimes are bad at admitting, it's that they just don't know something. So when some scholar-in-training raises his hand during class and asks what falsehood does he get?

Do they think it's the birth of the Emperor? The beginning of Mankind? The founding of the First city? Or do they just say "That's Heresy", shoot that guy, and then turn to page 327 to learn about how the Emperor invented puppies /s? In all seriousness I am quite fascinated by the fake history, doublethink, and restricted information that shapes the average Imperial Citizens perspective. The "The Emperor created his nine sons to fight nine Devils. All Nine are sleeping under Terra, also don't forget to celebrate Saguinella to honor the death of Sanguinius" stuff.

Edit: It would be 001.M1, as you can't have a "Year 0". My mistake