Oh, dear, that old bugbear. We'll have to discuss the Great Dating Controversy at some point in the Book Club, but I really don't think you can sanely twist your way out of the fact it's really rather hard to square the idea that
Alien Bodies lies 450 years in the future of the
Book with… well, anything, from a young Justine's presence to Mictlan still being around.
One solution some have put forward is to say that the present-day Dr. Who interacting with the War in
Alien Bodies did sufficiently change history that the War we see thereafter is a significantly different version from the one glimpsed in
Alien Bodies, one which developed much faster. Another might be to mumble something about dating systems — "are we entirely sure in
whose years either book was counting when they said 50 and 500?" and so on. And yet another, which I tend to favour, is to say that the
Book is lying about the 50-year figure, or at least creatively reinterpreting the truth. Insofar as the
Book is written with a human perspective in mind, perhaps the editorial decision was made to collapse every decade into a single year, because fifty years relative to a human lifespan gives the right
idea of how long a five-hundred-year war
feels like it's lasted to the beings on either side: i.e. a significant portion of their average life-span, but there are still many people who were in the War on Day 1 who aren't dead yet, though they're fast turning into relics of a bygone age.