According to my knowledge, I have not seen or heard any mention of these years when I played the games unless they completely escaped my vision/hearing somewhere.
In X's case, do you have a screenshot where the year is displayed in one of the cutscenes? I can't find it.
I'm going to look into 2 and Legacy because I swear, there was something that confirmed the year for that game, at least by the end operation.
As for X, you're right, I was mistaken. 2020 doesn't appear in the cutscenes, I can't find it either. However, I did find sources, which I'm going to add to the war's article; IGN states 2020 in an article which is dated 2006. Which is a good thing, because the pages for ACX and the Aurelian War weren't made until 2007, so they got that info directly from Bandai Namco back then.
That's one issue solved thankfully.
EDIT: I replayed Fighters Honor, still no mention of year. I don't think it's on the game anywhere. Is there an article like the one you pointed out which states the year Legacy takes place in?
In the Ace Combat 2 article, the Plot section says "May of 1997" but can that information be considered canon? I say no because in Legacy the war starts in March.
I'm really annoyed by this. I know there was a source for the game taking place in a specific year, but I have no idea where it is.
The biggest problem is that if we decide that there's no canon date known and that the rebellion took place in either 1996, 1997, or 1998, we'd have a lot of updating to do on every article related to the rebellion since they all state 1998.
But I am so angry... I know there's a source and I can't find it.
If you look closely, you'll see "Anderson Crater" and another named crater whose name I can't read clearly.
If we remember correctly, Ulysses 1994XF04 hit the Earth on June 3 1999, which means that the craters formed must have been named after that date.
Why are the craters named in this wallpaper? Is this implying that Legacy took place after the collision?
The craters are named because it's using the same exact map as File:Strangereal Map.jpg. They didn't bother removing any of the crater names.