Marvel Cinematic Universe Timeline

The Avengers (2012)

MCU Year: 2011~2013

The entire film happens in approximately two days with one flashback to 1945. That's the easy part. The complicated part is that this film is in limbo between the years 2011–2013. I have found no solid proof that it exists in only one of those years—Avengers: Endgame confirms (in title cards) that Avengers takes place in 2012.

The popular theory is that Cap was only a few weeks out of the ice when aliens invaded which is why he seemed so lost in Avengers and why he still dressed and groomed like it was the '40s. Though he was discovered in the same week as all the other events, it took time to extract him carefully, especially in extreme temperatures (think archeologists painstakingly uncovering ancient artifacts). Then there was the time needed to transport and revive him. This could feasibly have taken a year if not longer. But I just can't see him taking two years to revive or spending two years dawdling around in a fast-paced world. Steve woke up stunned and confused, sure, but he's also willing to jump headlong into challenges. He's not going to just hide from the 21st Century for two years when he's already ten years late for it. Anyway.

Other reasons Avengers takes place at least a year or two after the previous films is to provide enough time for:

— Bruce Banner to travel to India and establish himself as a doctor. {Natasha: "You've been more than a year without an incident."}

— Tony Stark to buy, remodel, and move into Stark Tower; which is powered by the technology he invented in IM2.

— Loki to be inducted, trained, and sent to Earth by Thanos. {The Other: "He is ready to lead."} 

— Thor to find a way back to Earth to catch Loki though the Bifrost is broken (Loki: "How much dark energy did [Odin] have to muster to conjure you here to your precious Earth?")

— Selvig and Hawkeye to monitor the Tesseract

— SHIELD to develop weaponry from the remains of the Destroyer (Coulson: "Even I don't know what it does.")

— 500 other reasons...

Other factors to narrow down the date are the condition of foilage in New York (green), the clothing of the extras (light), and the amount of sweat on the rescue personnel  during the battle (much). The battle itself was partially shot in Ohio in May–June of 2011, so it was a warm month.

Though it lacks convincing screenshots to prove what year it's in, Avengers can be loosely settled into 2012 by the timeline locations of the following films/shows/extras. 

Conclusion: May 2012

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