Given the, um, chequered writing history of AFFC I'm pretty sure George would not have been going around saying in print that he expected to be finished in a year if he wasn't very, very sure of that.
What we know is that he has at least half of ADWD (which, by the way, is not AFFC part 2 any more than ASOS was ACOK part 2) in a form where it's ready to be published. (Heck, three chapters of it can be bought already in a preview booklet.
And several more have been read at cons over the last few years.) Although these chapters are vulnerable to being rewritten, in theory, from what George has said this is unlikely: he knows how their plot arcs end already.
We also know that he hadn't planned to do any substantial writing on ADWD until this month at the earliest, given the touring and such for AFFC. So he's on schedule there, because he hasn't.
(Though he has almost completed the next Dunk & Egg story. Now he just has to find somewhere to publish it... shouldn't be a huge problem, tho.)
Finally, we know that the publishers want to release it pre-Christmas if humanly possible to maximise sales, and remarks George made on the tour suggest he thinks this is realistic.
The major possible delay is ironically if ADWD turns out too short. George has said that events in ADWD will extend a few months past the end of AFFC, and that if the manuscript is short enough he'll bring the AFFC POVs up-to-date as part of the book. This would not be wholly new writing, but opens up the possibility of delays. OTOH, it's not totally clear whether this will happen yet, so don't panic.
I'm pretty sure he'll aim to meet the original deadline as his top priority. And he did manage to write the whole of ASOS in less than two years...
