Your editor says the first draft is due in December and the published book will be on the shelves a year later. So you actually have some fudge time, don’t you? You can be a few weeks (or a few months) late getting the book in, right?
No. You can’t. First draft is just the beginning of the process of getting your book ready for publication. Once you’re done with it, the editor will read it—and she needs some leeway on the time it will take her to do that, because yours is not the only project she’s working on.
She’ll make revision requests (more on those in the next section). You’ll need time to do your rewrites. She’ll read (and we hope approve) your rewrites, then send the book to the copyeditor. The copyeditor will work on a tight deadline, and get the book to the compositor, who will set in into type. At some point in this process, you’ll get typeset galleys to go over and proof.
Keep your rewrites to the bare minimum at this point—it costs money, and usually a lot of it, to reset typeset print. Look for typos, things that are just plain wrong, and typesetting errors (like the last word of a sentence orphaned at the top of a page, or a place where lines were duplicated or inexplicably put into a different typeface.)
You’ll do galleys on an incredibly tight schedule; I’ve had turn-arounds of one day before. The galleys go back, the compositor finishes setting the type, the proofs are sent to the printer, the pages are printed and bound, and a book emerges. In this process, too, there has been cover design and marketing work going on simultaneously, and perhaps the preparation of bound galleys to go out to reviewers (usually prepared from the same galleys that you proofed).
All of these things take time, and the one thing that will screw up every single link in this long and complicated chain every time is you being late with your first draft or revisions. Take the deadline your editor gives you as being chiselled in stone, handed down from on high like the eleventh commandment. In fact, for writers, it IS the eleventh commandment.
THOU SHALT NOT BE LATE WITH THY BOOK!
If you are, assume that you will not be on terribly warm terms with anyone at your publishing house for a while thereafter. If you have a thereafter.