There'sAs said, the State of the Art and Consider Phlebas give the keys to matching up the Culture timeline with ours and then the (in my opinion somewhat vague) references within the rest of the books can be cross-matched to give a rough timeline that follows.
For reference: someone else has done most of the Gregorian calendarhard work and published it here on Wikia: Timeline on Culture wikia follows the Gregorian calendar
Have to admit I've not cross-checked it myself (currently working through all the books in publication order (more or less) but only at Inversions so far).
I don't believe there's any special significance between the Culture timeline and our own - indeed, according to State of the Art, we're not even worth contacting.
The second part of the second question is answered in the same story, The State of the Art - the Culture visits earth, and since the Culture is pretty much always referred to as being in 1 galaxy only, by implication, we are both in the same galaxy: the Milky Way.