It wasn't canon - many dates and events conjectured in the chronology were changed when they eventually made it to screen. There's even a disclaimer to that effect at the start.
What had been made canon with regard to TOS' timeframe is...
The five-year mission ended in 2270 (VOY: "Q2")
Kirk was born in 2233 (STXI)
Kirk was 34 during "The Deady Years"
A deleted scene in STXI gives 2230 as Spock's birth year.
I think it all works out close enough to the Star Trek Chronology's conjectures. There's an updated version of the timeline in Voyages of Imagination
, which includes all the films, episodes and novels up until about 2006. It includes the updated info from "Q2" and thusly moves The Motion Picture to 2273.