There is no mention in the book (so far, and I have read pretty much all the background in the new rules) of what "year 0" is. It was never something they made a great deal of - but it was always presumed to be the birth of Christ. Which, frankly, seemed odd - I can understand the use of that dating system for thousands and thousands of years, even by people who don't believe in Him (it is just a legacy system and one almost the whole world agrees on). But after something like the Age of Strife and the battles of the techno-barbarians? Well, you would think there would be different dating system - one stemming from the Emperor's interment in the Throne, for example.
Then again, how many people really need to know the galactic year? Local dating systems are probably far, far more important.
Not gonna lie, messing with the timeline is almost always a shitty plot device.
But they aren't playing with it for any reason I can see - they aren't (for example) saying it is the late M41 so the Gathering Storm didn't happen, or anything. So far as I can tell, it is just a mechanism so this isn't Warhammer 41K . . .
The other thing with the timeline of 40K is that time flows differently; Warp travel messes everything up, and the Cicatrix Maeledictum and the great storms lasted "for weeks or for centuries". I really think there *is* no normal temporal progression - perhaps deliberate to allow you to field whatever armies you like.
On another note... I can find no reference in the new rulebook to the
destruction of Cadia. Its reduction to a wasteland, but nothing about actual destruction . . .