User blog comment:TUSF/Light Novel Chronology/@comment-1173941-20121227131055/@comment-4631119-20121228053459

For the Floor 28, it's mentioned on the anime, at the scene where the whole guild is resting, and also at the scene where Kirito sneaked out at night to hunt and gain some experience, when accidentally he meets Klein and his gank.

For the Floor 30, it's stated in the novel (Red-Nosed Reindeer, part 2). Here I'll quote you some:

"The five of us entered the dungeon which we never went to before, the one which was just below the 3 levels the frontlines were at . Of course, I fought in that place before, and I knew that it was a place that was easy to earn money but had lots of traps. However, I didn't tell them that."

If you remembered well, in the anime it's stated that the dungeon is on 27th floor, so at that time the frontline should be 27+3 = Floor 30.

For the floor 49, I do think you already know the reference, since you posted it before, but I'll referenced it here as well, since I also put that floor in the list:

"Having said that, after continuously battling for an hour under this weak torch light, I did indeed feel my concentration waning. I can no longer react and counter their big, biting jaws or acid mucus as well as earlier. Though they attack in large numbers, these monsters are not small-fry. This habitat is only three floors below the front line on the 49th floor, and these are very powerful monsters. Even though this is within the safety margin when considering level differences, if a massive swarm were to attack and surround me, my HP would quickly go down to the yellow zone."

For the timeline, it's all anime-based.

That's all the reference, from where I got these information. Hope it'll be helpful :)