In the book's english edition, the epigraph of chapter 43 dates it as 25 September 3028 as well.
The NAIS 4th Succession Wars Atlas is, as an in-character document, a treatise by Dr. Banzai about the war ("sourcebook fiction").
The novel, by contrast, is narrated from an omniscient viewpoint ("novel fiction") and as such automatically trumps the sourcebook as per the official ruling in the 25 Years of Art and Fiction tome (page 6).
That said, either source may simply be in error in the sense of a typo. In which case the "novel fiction trumps sourcebook fiction" and "newer trumps older" mantras don't apply. :)