Citing work with a publication year in the future

Most journals take as formal publication date the date it appears on their printed version. The online version is considered a pre-publication.

Thus, despite how weird it may look, it is ok to cite an article from the future (as long as it is accepted and in press or the online preview). Journals with only online presence would not have this issue or the time span might be smaller.

What is probably important is to provide the DOI of the article (if available) as this will make it easier for people to find its online version before and after it has been published on paper.

Edit: Just to add that the citation and its details is decided/provided by the journal and not by you.

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.