If you publish a book January 1st 2018 can the copyright year still be 2017?

@steve717 A work is copyrighted from the moment it's created, so in general there's no such thing as "you copyright". In the US you could register it with the copyright office for proof, though that's not really a necessity, and the act of "publishing" is where people will know you authored it. "The year of publication may determine the length of the copyright term for a work made for hire or an anonymous or pseudonymous work." (US Coopyright Office). The year in the copyright notice is "The year of first publication of the work (or of creation if the work is unpublished)" (Same source).

@Faust76 agree with you except in cases where others would try to steal your work. I always register my work. Even though some would say it's unnecessary. Per your information here. It is precisely in agreement. Copyright registration usually can definitely occur before a work is published. Which is why a work can be copyrighted in 2017 and published im 2018.

@steve717 Well I mean, to stay on the topic of the question, it kinda depends on what is meant by "copyright year". If that refers to the copyright notice, as one would usually expect, then by law it's strictly the publication year; if one wants to backdate that then the only legal way is to make a limited publication, of possibly slightly different work, before. But a publishing house would likely not like that, as they want the prestige of first publication.

I'm no lawyer and not giving legal advice. In todays world you'd have to consider the laws of every country in the world, as there's no way the work could be restricted to, say, USA only.