Cani i ask where you got those dates from (the 2179 one does sound familiar). I ask because Cameron said he set AVATAR in the smae year as ALIENS as a little eatser egg for the fans, kind of making out that the films are in the same universe, anyway the year on the AVATAR cameras that Jake speaks into are 2150 something (2153 i think).
The crew of the Prometheus comes out of cryosleep on December 21, 2091 according to the subtitle in the film giving the information on the Prometheus and its crew and its distance from Earth.I'm guessing this isn't set 30 years before ALIEN then. ALIENS is set in 2150 something (can't remember the exact date) and Ripley had been in hypersleep for 57 years meaning that ALIEN is set around 2100.
Alien is set in 2122 and Aliens is set in 2179. This movie is precisely 30 years before Alien.
I believe the dates are derived from Aliens, specifically Ripley's line: "I just checked the colony log, directive dated 6/12/79, signed Burke, Carter J." I'm guessing that the year Alien is set is backward derived from that, hence 2122. That Prometheus is said to be set 30 years before Alien, and is definitively stated to be 2092, further corroborates that.
I could be mistaken though and the 2122 date is derived from Alien, either its script or novelization and Cameron got his dates from that, but I don't know.