Marvel has reinvented the entire world and mythology with writer Jonathan Hickman's "House of X" event series, setting the stage for a full-on X-Men reboot in this week's "Dawn of X" launch. House of X wasn't just the introduction to a new world of X-Men - it was also a much-needed tidying up of the franchise's extremely convoluted continuity. Hickman largely succeeds in retconning much of the X-Men continuity in excitingly fresh ways - but some fans have been highly-critical of the new Marvel's new X-Men timeline, and how the math of it adds up. Now Jonathan Hickman himself is speaking up X-Men's timeline, and his attempts to straighten it out.
According to Hickman himself, House of X was far from perfect in terms of establishing a fully-accurate X-Men timeline. While doing an interview with AIPT, Hickman was specifically asked about his decision to cram several big X-Men events into just a three-year period of the X-Men's lives:
Q: "Can you explain why you’ve smashed Genosha, House of M, “Messiah Complex,” the Utopia period, the Schism, all of Wolverine and the X-Men, Secret Wars II, Civil War II, IVX, and Uncanny X-Men vol. 5 into a three-year period for Moira X? It doesn’t make sense in a book so fixed on time passing." --Murphy Leigh
In response, Hickman acknowledges that the timeline doesn't make sense according to conventional measures of time - but it does work in terms of Marvel Universe time:
"You’re right. The math doesn’t work. The math also doesn’t work for a single other long-running book in the Marvel universe. I personally don’t believe that those things took place in that duration of time, but on a sliding continuity timeline, I’m also wrong."
If you're unfamiliar, a "Sliding Timescale" is a Marvel Comics storytelling device that posits that the flow of time within the Marvel 616 Universe (and related universes) occurs at much slower pace than the flow of actual time in over which comic book stories are published. It was introduced in the '70/80s ear of Marvel Comics, as means of explaining how characters can be in comics for decades without any noticeable aging. In 2007 it was established as official universe canon in Official Handbook of the Marvel Universe A-Z #2.
So, while even Jonathan Hickman doesn't quite agree with the parameters of a three-year timeline he had to create around the X-Men's story from the 2000s - 2010s, it does make sense on a Marvel sliding timescale. At the end of the day, it's probably best not to think too hard about it.
House of X and Powers of X are now done, with six issues of each book available for sale. Jonathan Hickman's X-Men #1 is also now on sale.