I got into a discussion with a friend about whether 40K is or isn't "stupid", and after countering a bunch of 3rd Ed. fluff (mostly concerning Orks because it always does because Orks were in actuality stupid before 4th Ed.), we got to something that I can't defend:
It's possible to enter the Warp, and exit from before when you left. I can't recall an instance of this ever actually happening (look, I'm very aware of John Grammaticus...Just...Ergh...I can't right now), at least not in a way that matters. Like; I guess it might have happened at some point.
But like...A Company or three of Ultramarines Warp-travels to the past by accident on Battle Barge. They are kitted out in full Primaris gear so it's obvious they're from the future. They know about the Fall of Cadia, and how to prevent the Great Rift, and they know about the resurrections of Guilliman and Lion, and they tell everyone how it's going to go down. Save Yvraine. Save the Galaxy. Then just like...Tell people where the Will of Eternity's shield generator is so the Space Wolves don't waste time looking for it. Warn Garadon that Shon'tu and Be'lakor are going to infiltrate Phalanx, in order for Phalanx to not arrive at the Fall too late.
Setting over. 8th Ed. and beyond just...Don't happen.
Instead it's like; A Company of Space Marines no-one cares about arrived on planet no-one cares about and then they all died. The end. We can't actually talk about time travel because it would literally ruin everything. But we have to get over how unpredictable the Warp can be and this is the most random thing we could think of, even though it has the potential to literally break everything.
Time travel to the past is setting-breakingly stupid...As I said, John Grammaticus...I just can't.
Something, something, Ordo Chronos? But as I said, you wouldn't be able to "Ordo Chronus" away three Companies of Space Marines with one or more named Characters, especially if they come bearing Primaris parts.