When Does "ALIEN" (1979) Take Place?
"ALIEN" is a horror science fiction film written by Ronald Shusett with Dan O'Bannon, David Giler, and Walter Hill that released May 25th, 1979. So when is "ALIEN" set?
It takes place on the fictional Alien vs. Predator timeline in the year:
We know this because during "Alien: Romulus" (2024), which takes place in 2142 AD, Science Officer Rook specifically says, "One of our towing ships first made contact with this specimen 2 decades ago. Of the seven crew members aboard the USS Nostromo"...

2122 AD
We know this because during "Alien: Romulus" (2024), which takes place in 2142 AD, Science Officer Rook specifically says, "One of our towing ships first made contact with this specimen 2 decades ago. Of the seven crew members aboard the USS Nostromo"...
2 decades = 20 years
and
2,142 - 20 = 2,122.
This StackExchange user's post examines its exact setting.
It is a midway chronological film in the Alien vs. Predator timeline (the chronological fourth film to feature an alien xenomorph) and can be watched either at its timeline point or entirely separately.

