King Daniel wrote:
It was in the early 90's when TNG was huge, that the Okuda's Star Trek Chronology was released, which moved around all the dates, rendering the prior book obsolete. The writers of all subsequent Treks switched to this new dating system.
It's worth noting that the Okuda chronology was extrapolating from what had then been firmly established onscreen. The TNG season 1 finale, "The Neutral Zone," gave Trek's first onscreen mention of the current date in Earth years (2364).