Which is why I have been absent without leave for so long. Deadlines come first.
The deadline is the appointment you have made for delivering your manuscript (or corrections to it) to your publisher. The key phrase there is 'you have made' - you agreed to this deadline, so you have to meet it. It is impolite to miss an appointment you've made, and it is deeply unprofessional to miss a deadline you agreed to. End of.
If being professional and polite aren't that high on your list of priorities, you probably shouldn't be reading this blog as it is for people who want to be professional writers and I prefer polite people. But if you feel a lapse once or twice won't matter too much to your overall image, let's look at some other consequences. I suppose these are really the explanation of why it is unprofessional and rude to miss deadlines.
You are not working in a vacuum. When you deliver your manuscript, an editor will read it. If you are delivering corrections, a copy editor is waiting for it, possibly an illustrator and a designer. Some of these people might be freelance and will have set aside time to work on your book. If your book doesn't turn up, they will have wasted time that no one will pay for. They still have to do the work on your book when it comes in and will be paid the same, even though they spent a day watching re-runs of Futurama or weeding the garden. Then it will have a knock-on effect on their other projects, or they will have to work over the weekend to get the book back on schedule. Shame on you. Even if the next people in the chain are not freelance, they will be (quite reasonably) annoyed if their own work plans are disrupted by your inefficiency. Those others in the chain are people, not just editors - remember that.
If you are so late the book can't be dragged back onto schedule, the publisher might miss the slot booked to print the book. At the moment, printers are trying to claw their way out of the grave they've been pushed into and might be able to accommodate your book anyway - but don't depend on it, and don't depend on things always being that way.
If you deliver late, there will be less time for everyone else to work on your book, so there's a good chance it will be a worse book. Those people aren't messing with your book for the hell of it - their job is to improve it. Yes, it can be improved.
What if you really can't meet a deadline? First, work out why. Here are the possibilities:
1. A personal disaster has befallen you - someone close to you has died, your partner has left you, your house has burnt down, someone close to you is very ill and you are their carer (or shared carer), you have fallen ill (properly ill, not man-flu)
2. A technological disaster has befallen you - your file has become corrupted or been deleted, the laptop it was on has been stolen, your computer has been disabled by a virus
3. You've simply run out of time - you mismanaged your time and there is too much left to do, what you have written is rubbish, what you have written doesn't match the brief/outline, the structure you picked isn't working, you got scared/blocked and couldn't carry on.
And now the solutions/stern chat:
1. Not your fault. No one is likely to hold you to the deadline. Tell the editor as soon as you are able to or, if you are not able to, get a friend to email your editor and alert them to the problem. Then you can forget about the book for now and do what you have to do.
Don't do this if you have a cold or something short term. If you have broken your leg, that gives you a few days but unless you are an armless person who types with your toes, it won't stop you finishing the book after that.
2. Use the back up. You WHAT? You don't have one? You can now leave the blog - this is for people who want to be professionals.
If you don't have a recent back up, stay up all night recreating the book from an older back up and consider it a useful lesson. I recommend you don't tell your editor you lost the file - they will think you are a bozo (and they will be right). A virtual dog ate my homework. The physical lack of a computer might cost you a day while you track down a library where you can use one, or one you can borrow or lease. You can probably catch up that day, but if not you must explain to the editor what has happened and how you are solving the problem and how long it will take (no more than a day, remember)
3. This doesn't happen the day before the deadline - you see it coming. If you really, really can't recover the situation, you have to talk to your editor as soon as it becomes clear to you that you can't meet the deadline. The editor wants your book to work - it's a lot of aggro if it doesn't. You can ask them to help you by reading what you've done and suggesting a different structure, or how to get it back on track, or whatever is needed. They won't be happy, but they won't be as unhappy as if you don't do this and just don't deliver, or deliver rubbish.
It will not be the end of the world - they will help you, or fire you. They won't send round a hit squad or kill your children. To them, it's one of many books - it's only the centre of the universe to you. If it is commissioned non-fiction, they might get an editor to fix the mess. If the mess is too messy, they might send you a kill fee and employ someone else to finish or rewrite it. I've done many a fix after a kill fee and I've also rewritten from scratch books that someone has screwed up or pulled out of. The industry has strategies to cope. It's more of a problem if you're writing a book someone else can't fix and it's in the catalogue already. Don't expect to be popular, but they will try to find a solution. For their benefit, not yours - it's not a favour. Don't ask them to publish your next book.
Things you must not do:
Lie about a major disaster that hasn't happened to you. How many mothers can you afford to kill off in the course of your career? Besides, now everything is transparent and you don't know who knows whom. You tell your editor you have to go to your mother's funeral and it's clear from Facebook you're shopping in Oxford Street. Your editor may not be your Facebook friend, but their flatmate/partner/parent/child/friend might be: 'My poor author, Stroppy, can't deliver because her mum died'. 'Oh, it says on her Facebook page she's going on a picnic with her daughters.'/'Oh, I know Stroppy - I'll send her a card.' See what I mean?
Just go off radar. Not responding to email and not answering the phone - for more than a day, when you might plausibly have network problems or be out - is cowardly and unprofessional. And unimaginative. You can do better than that.
Sending a garbled file (if you know how to make one - if not, learn). It can buy you a day at most. The editor should open it immediately to check it's readable (ie to check you haven't deliberately sent a garbled file) and then they will email you to ask you to resend it if they can't. If you're going to say later you were out, don't hang around on twitter saying 'yay, I finished my book, I'm going to have a coffee and read the paper'. I don't recommend the garbled-file route as you can't tell how much time it will buy you. If you only need another day, it will do. More than that, it's risky. If you have a trusting/lax editor they might not check the file until they want to start work on it, and that might be a week away. Then they will be embarrassed that they've left it so long, and you will have got lucky. But it might be tomorrow.
Make stupid excuses. Either be honest or be quiet.
On the whole, the more urgent the deadline, the more important it is to meet it. The deadline for an academic book may be years away, and in my experience the publishers will be so surprised if you deliver on time that they'll assume your book/contribution is not very good. That doesn't apply to real-world publishing, so if you're a refugee from academia, you're in for a surprise.
Oh, and one last thing. Your editors will really love you if you reliably meet deadlines. Which is stupid, but just goes to show how many people don't.
Now - don't you have a book to finish? Reading blogs is no excuse for missing your deadline!