Deadlines are funny things in the book business. I started my professional writing for weekly magazines - there, the deadline was clear. My copy had to be in by 5pm on a particular day. There was no flexibility. Either it was in on time, or it didn't make print.
But with books, the whole thing is more handwaving. There is a set date. A specific date - not just 'April', say, but 26th April. But because of the timescales involved, there's much less pressure to be precise about this. With those weekly magazines, my copy was going in less than a week before the magazine came out. With a book there's often a year between my manuscript going to the editor and the book being published. Inevitably things are rather looser.
I've even heard it said it some quarters that publishers can be a bit suspicious of people who always deliver on time as they're clearly writing to time, rather than getting it right.
However, I do usually make it. (And then, often the manuscript seems to sit on the editor's desk for a few weeks because there are more urgent things to do.)
My next deadline is not until October. At the moment that seems a long way away, which works because I'm mostly researching and just writing bits here and bits there at the moment. There's a temptation to be complacent about it. But my magazine background keeps nagging. 'Have you done the n words per week you need to have it finished a month before due date?' it asks. So the chances of me delivering late are pretty small.