When you send a publisher a book proposal, you need to propose a date — the date by which, if the publisher contracts the book, you would deliver the finished typescript.
Unless you are writing a textbook, it is unlikely to matter much what time of year you propose for delivery (textbooks are different, because their sales are heavily seasonal).
Meeting the delivery date is important, because publishers build other things — principally revenue budget forecasts — on it, so it is important to ensure that you can meet whatever date you negotiate for the contract.
Be cautious: if you deliver before the date in the contract, nobody will object; if you fail to deliver by that date, you may be causing the publisher a problem. In the worst case scenario, you may find that your contract gets cancelled.
Be especially careful if your contract contains the phrase ‘time is of the essence’ — that is a sign that the publisher is serious about dates.
At the outset of the project, it’s easy to let enthusiasm sweep you up and commit to an early date. The problem is that such dates tend to prove unrealistic. Stuff happens: people fall ill, or move jobs, or move houses, etc. So go easy on yourself: cut yourself some.
Unless it’s a large book, such as a full-length textbook, most serious books, such as monographs or how-tos, seem to take about a year. I’ve learnt as a publisher that if someone has a longer delivery date than that, the author tends to just get started later. Less than a year, the likelihood of late delivery increases.
If, having signed up to a date, something happens — a bereavement, for example — to delay the writing, it’s best to contact the publisher to discuss the schedule. Always keep them in the loop.
In the actual proposal, propose a date in the form of ‘contract + x months’, rather than an actual date. That way, if the publisher takes a long time to commission the book, their delay doesn’t put you under pressure — the delivery date just gets pushed into the future, so the time you have in which to write the book doesn’t get squeezed.