There’s no way to make Iron Man and Avengers within one year of each other (the maximum deviation say Homecoming can at least be 7.5 years after The Avengers and Civil War at most 8.5 years after Iron Man), and so it cannot be consolidated.
As it’s also set months after The Avengers, we can place the prior movies in 2010 and 2011.
With Iron Man 3 we get the first explicit dating in the movies: it’s set at Christmas 2012 (thirteen years after New Year’s Eve 1999).
It used to be that every Marvel film was, unless explicitly stated otherwise, set in its month of release.