system
Given the following post, respond as if you're commenting on Reddit:
human
[Avatar:2009] [Alien] If the movies of Avatar and Alien are in the same universe how would you link them with a timeline?

Let's start from the events of the pc game and end at the latest movie of the alien franchise. (You can edit any event inconsistencies within both franchises and add any more events to make the timeline more detailed/consistent)
gpt
Are they really?  

The main problem is that *Avatar* takes place in the mid-22nd century, and they use sublight engines to get to the Centauri system.  

*Alien* takes place in the early 22nd century, but there is already an extensive interstellar industrial and economic infrastructure in place complete with common faster-than-light travel.  They travel to Zeta II Reticuli, which is roughly 30 light years from earth and approximately an 18-month transit time; it's mentioned in *Avatar* that it takes the better part of 7 years to get to the measly 4.3 light year distance to the Centauri system.   

That makes it difficult to reconcile both universes. Your best bet would be to create parallel timelines that are linked in some way, perhaps with Engineer tech, or as an anomalous property of unobtanium; perhaps one universe is a simulation being run in another.