Since they've been posted so much, I would assume everyone's seen the Kids OVA, but I'll cut this just in case...
Outside, the grass is green, and the trees are in bloom...so it's either mid-late spring or summer.
But wait...Ed's birthday is in the winter! WTFH?
So, plausible reasonings? I doubt they're south of the equator. Do we have a plot hole on our hands, or is the Gate screwy with the timeline between worlds and Ed just adjusted his birthday accordingly or something like that?
And other comments regarding it while I'm here....
He's in damn good shapre for 100 too...still got color in his hair even (curses mother's lineage for the prematurely greying genes....and ironically, my dad's side is the type that DOESN'T get grey; he has very few at 50, and even HIS dad still didn't have too much greying into his 70's...)
Not to mention that automail is 80+ years old, and still functining...Winry would be proud XD
In the past, I've seen people wonder how his grandkids are such little look-alikes. My mother cousins daughter whom I met like 10 years ago at a reuinion, looked EXACTLY like me. If it weren't for the nametags, no one but our parents could tell us apart. We were only blood related though our GREAT Grandparents. I also look very much like my father, who isn't even related to her at all. So while it's not exactly common, you CAN have little clones running in families, even generations later and with mixed bloodlines.
Okay, so it's a longshot to have it happen like that all at once, but it's anime...they break the rules about this crap all the time.
I'll shut my trap now.