Homecoming (which has its own issues) just has Happy referencing having the ring for 8 years, while a reference to the movie, it doesn't necessarily mean he's had it since the events of the movie. I 100% believe Iron Man was originally intended to be 2008. However, with Iron Man 2 being 2011, I just personally can't see it as being 2008. Not only is there the 6 months later title card, but Hammer mentions how Tony has created a weapon "in the last 6 months" and Pepper mentions Christine doing "quite the spread" on Tony last year. I feel like it's a huge stretch to have 3 years between them. Of course, there is evidence for 2008 and 2010, so until we get further evidence, it's up to the individual to decide what they want to go with.