Assuming that TOS events in GEN take place in 2293, the
Enterprise-B is the first starship of that name in "30 years" without Captain Kirk.
This would suggest that he assumed command of the TOS
Enterprise in 2263 (Stardate 1277.1 ?).
Around Stardate 3579.4 ("The Deadly Years") Kirk claims he is 34. Assuming 1,000 stardate digits equal one year would put events in that episode somewhere in or around 2265, minus 34 = 2231.
Events in "Space Seed" (Stardate 3143.3) would have taken place in or around the same year, plus 15 years = 2280.
But according to TWOK
KIRK: 2283.
McCOY: Yeah well it takes this stuff a while to ferment. Here now, gimme. ...Now you open this one.
Another approach could be to assume the 5-year-mission started at Stardate 1277.1 and 8 years later it's Stardate 7412.6 (TMP). One year would equal 767 stardate digits.
This would put "Space Seed" in or around early 2266 (DY in 2266), 15 years later would still only be 2281.
Looks like the later birthdate could help explain some things a little better, IMHO.
Bob