Yeah... there's no reason Retaliation has to occur only a week and a half after The Rise of Cobra from what I can tell. I didn't spot a 2015 date at all either.
The only dates (that I can verify) are:
G.I. Joe: The Rise of Cobra = August, in the near future
G.I. Joe: Retaliation = pre-2016, one month before Global Warming summit
So, I think if there's no 2015 date, August-October 2013 is suitable (Retaliation released in 2013). There was a UN Climate Change Conference in November 2013, so one month prior is October, and then you can match the time passage shown in the Operation HISS comic to bring it into October.
Funny enough, if you wanted to connect to Transformers, that makes things easier. With dialogue at the end which states "with the world back in order, rebuilding has begun around the world", there's an implication that London will be rebuilt, and there'd be a 10 year gap between this film and The Last Knight.
Which would lead us to this.
300mBC - Transformers One
1987 - Bumblebee
1994 - Transformers: Rise of the Beast
2000 - 28 Days
2007 - Transformers
2008 - Friday the 13th: Killer Cut
2009 - Transformers: Revenge of the Fallen: Big Screen Edition
2013 - Transformers: Dark of the Moon
2013 - G.I. Joe: The Rise of Cobra
2013 - G.I. Joe: Retaliation: Extended Cut
2018 - Transformers: Age of Extinction
2023 - Transformers: The Last Knight