JSA SF&O #1 (1999) ["I am eighty-six years old."] has to be countered with the SMT #2 narration that has Wes's mother, whom he barely remembers, dying while his father was away fighting in World War One. This seemed the best compromise, although obviously both Wes and Dian are meant to be older.
It's the birth year on her tombstone in All-Star Squadron #18, although obviously rumors of her death were greatly exaggerated (thank you, Mr. Robinson). It fits with SMT because Dian is a recent university graduate in 1937. As with Wes, it's near impossible to rectify their amount of life experience in SMT with their necessary youth in order to be running around in the 1990's - Ian Karkull and the Ragnarok cycle being of no help here.
Sandy's true origins are unknown. His status as Dian Belmont's nephew is in question after repeated SMT references to Dian being an only child. His age is also wild speculation - the canon tells us that Sandy was transformed into a silicon monster (the Silicoid Gun Incident) in 1947, but never says how old he is when that happens. Fanon put him at about fifteen, but there's no evidence (apart from a few youthful-looking appearances) one way or the other and, in fact, there is much to suggest he is older - or at least he should be older - than that.
Sandy was active - and well-experienced - as the Golden Boy in 1944 [various sources; JSA #18 the most recent] and if he were fifteen-sixteen in 1947, that would make him twelve-thirteen here and that's just too ridiculously young. The JSA would have never stood for Wes letting Sandy tag along even if Wes didn't have his own reservations [SMT #50 - "Can you believe that? Who would put a child's life in danger?" as he and Dian read the comic book account of the Sandman and his kid sidekick, Sandy 'the Golden Boy' Hawkins.]
Aging Sandy a few years is therefore pretty necessary, but it must be capped by the fact that he's not fully an adult by 1947 (I'm not a fan of Monsieur Aucoin's artwork in JSA #5, but he does make Sandy seem quite a bit older than fifteen when showing the accident). Setting Sandy's birthdate in 1929 is a compromise and a guess. It makes him fifteen during the Johnny Sorrow affair in 1944 (JSA #18), fifteen-sixteen during 'The JSA Returns' and 'The Last Days of the JSA' (both set in 1945), and puts him at around his eighteenth birthday in time for the fateful encounter with the silicoid gun in 1947.
[Events are told, sans date, in JLofA #113, DC Presents #42 and DC Presents #47.] The choice of 1992 is one of convenience - it makes for a nice round number in terms of Sandy's imprisonment (forty-five years) and works well enough with the "Nine Years Ago" from JSA SF&O #1 (1999).
Just not when DC says they happened. I'm not buying that it's been five years since the (1986) Crisis - you don't compress seventeen years of comic book happenings into five years - and two years since Zero Hour. I'm not even sure the writers buy it - this sort of compression has Pieter Cross taking over as Dr. Mid-Nite before Charles McNider has been killed. (While that's not technically impossible, Dr. Cross knew Dr. McNider - and apparently knew of his alter-ego - and Pieter's just not that sort of guy.)
JSA #1 came out with a cover date of August 1999 and the JD epic disaster was the team's first crossover.
Matching up the titles: JSA #1 ~ Green Lantern #113 ~ Starman #61 ~ JLA #33 ~ Early-Mid No Man's Land.
I nearly broke my brain on this: In JSA #1, Alan Scott makes reference to Jen Hayden's recovery from her stint as Green Lantern (Kyle Rayner returns to rescue her from Fatality in GL #111-12); Starman #61 contains references to JSA SF&O #1; JLA #35 takes place after Judgment Day, which makes it too late, and JLA #31 features the elder statesman JSA members (the end of 'Crisis Times Five') in a team-up that obviously predates the re-founding; JLA #32 and #33 both involve No Man's Land and events necessarily make it early on in that story.