"BBY/ABY" evolved from the earlier "BSW/ASW" ("Before Star Wars" or "After Star Wars") as a
real-world way of keeping track of when events occurred relative to the zero-point of the first film. By the time of the Prequels, the EU had exploded beyond the seven books and one comic series that were around at the time of the Original Trilogy. The role-playing game, video games, anthology collections, Heir to the Empire, Jedi Academy, Shadows of the Empire, Dark Empire, Tales of the Jedi... There was a
lot more in the late '80s and early '90s. All the creators had gotten used to using BSW/ASW and having the first film be their dating referent. So they just shunted it across in-universe as the event at the climax of that film. It could just as easily have been BDA/ADA (Before the Destruction of Alderaan/After the Destruction of Alderaan) or something for Tarkin's death, or Obi-Wan's. Whatever.
Point is, like so many things in the EU, not enough thought was given to it at the time of its creation. One thing I'm actually hoping is that the Story Group goes back to primary sources, starting with the first film, chronologically, and going from there to determine stuff. Like reclassifying the troops on Leia's ship. They've been called "Rebel Fleet Troopers" for so long, by so many people, in so many places that it's not even noticed any more. But think about it. If they're wanting to pass as a consular ship ferrying an Imperial Senator,
why in hell would they have uniformed Alliance soldiers aboard? That's dumb, and beyond idiocy for the Captain to keep maintaining it to Vader whilst wearing said uniform and gasping out his last. Scrub the conjectures and assumptions and go back to what we can glean from what's in the actual
films. Then go from there.
My take on those troops is this: Their helmets are similar to the helmets the black-uniformed Imperial soldiers are wearing (Stormtrooper enlisted personnel in garrison uniform, BTW

-- same process of deduction). Perhaps this is the time period's garrison uniform for the Imperial Senate Guards. And the Captain whatever the Judicial Fleet has become. Wors better than uniformed Rebels crewing that ship.
As I mentioned above, the original notion John Mollo had for the Imperial uniforms was the black ones were for the "SS" types -- the Stormtrooper armor is the battle dress, but the black uniforms are the garrison uniforms. And, like the Grand Army before them, and like the Marine Corps today, they maintain their own pilots and armored-vehicle operators independent of the Imperial Starfleet chain of command, but are subordinate to it. So the black-jumpsuited, Stormtrooper-faceplated TIE Fighter Pilots and black-uniformed, clamshell-helmeted Imperial Gunners are all Stormtroopers.
Or how about this solution to the change of ranks displayed from ANH to ESB -- Tagge and Motti are both listed as Generals in the credits. Tagge and Bast are seen consulting at the conference table, and sitting on the opposite side from Motti. There is clear animosity between Tagge and Motti. Bast is seen relaying the signal to fire to the gunners. So maybe Motti is the Army facility commander, Bast is the commander of the facility's/Tarkin's Stormtrooper garrison (Tarkin, incidentally, being what the TK represents), and Tagge having arrived on the
Devastator as the General commanding the embarked Stormtroopers (hence "Dangerous to
your Starfleet" spoken to an Army officer), a role Veers held on the
Executor. So the single-row rank badges are all Army badges, and incorporate all three primary colors in displaying rank in various ways.
In ESB we saw the
Navy mode of displaying rank, and Veers and the Snowtrooper Lieutenant adopting a two-row mode for ease of marking chain-of-command. Finer ranks than our contemporary O-1 to O-10 are used via non--visual means. All the plaques show are paired sets of low (one red and one blue), medium, (two of each), and high (three of each) line officers, embarked Commander of Ground Forces (four of each), and Fleet Command (six of each). Finer distinctions are unnecessary in verbal address (Ensign, Lieutenant, Captain, etc.), and each section aboard ship will have their own established internal heirarchy.
Naturally, each branch considers
their mode of rank display superior to the other's.
Don't get me started on the ranks in ROTJ.
Yes, I can see you warming up the tar and sharpening the pitchforks. Hold off on your knee-jerk reactions, though, and stop to think about it all for a bit. Let go of the EU and open your mind. Through the Force, things you will see...
--Jonah