I agree with @George. There's no problem at all with citing a publication date in the future. The more difficult case in when an article is available online now, but there's no publication date yet provided. Then I believe you should cite the online version, with the date that it appeared online, but change the citation if you can at a later time. For example, if your paper is finally being prepared for publication, the production staff might ask you to update citations of this kind with actual publication dates. If they don't, you should do so anyway.
There are some odd cases. Some books published and available for purchase late in the year may have a copyright date for the following year. A journal in my field once had editorial delays and ended up not publishing an issue dated one year until several months into the next year. So in March, you could cite a paper as appearing the preceding year--it was known which articles had been accepted--but you couldn't yet provide page numbers, because the journal issue had not in fact been published and page numbers weren't publicly available.
In all of these cases, you should use as much of the official publication information as possible, I believe, and follow George's other advice.