The good news is that most publishers will cut you a little slack if necessary. I didn't really understand this to start with. My first professional writing was for weekly magazines, and with them, the deadline was 5pm on day X, and that was it. Then on one book I misread the deadline as the last day of the month, where actually it was the first. When I was about two weeks late, the editor mildly asked if I would be ready soon. It wasn't a problem. In fact I've talked to academic publishers who reckon most academics think that if they get a book in within 3 months of a deadline, they've hit it.
That's excessive, but as long as you give them some warning, editors are usually happy to be a little flexible. For me, though, that magazine heritage is still there, and it's very rare that I don't deliver on the specified date. (In fact apart from that accidental overrun, I think I've only once delivered 2 weeks late by agreement.)
You might think that the pain of a deadline goes away once you've sent off your manuscript, but in some ways the worst bit is still to come. Now there's a pause. You know that eventually the editor is going to come back and say:
- This is great, we just need a few tweaks, or
- I do have quite a few notes, but it's fine or
- Okay, we have a problem here
I realize an editor isn't just working on my book. But I do think there could be a little more communication in this period. If (s)he knows a few days before deadline that (s)he isn't going to pick up your manuscript in the next month, it would be nice to be given some breathing space. If once (s)he's got it there is a delay that isn't caused by trying to work out how to salvage your book from disaster, it would be great just to get a little 'Everything's fine, I should get round to looking at it next week.' But on the whole there is very little communication.
It's not true of every editor - some really keep you in the loop, and/or are lightning fast with an initial assessment - but these are relatively rare. So perhaps the worst thing about a deadline is that you hit it not so much with a bang as a whimper. The big day arrives... and nothing happens for a month. Hey ho. Don't you just love deadlines?