Conference held in year X, proceedings published in X+2 - which to use? A certain conference was held on year X, but its proceedings were published in print in year X+2.

When I cite an article submitted to that conference (which I read in the published proceedings) - how do I indicate the correct year? 

I'm asking specifically about BibTeX field values, but a less-specific answer would also be ok I guess.
When I cite an article submitted to [a conference held in year X and published in year X+2] - how do I indicate the correct year?


The citation should include the publication year, not the year the conference was held. But, the conference year is useful, so it is worth including too. Perhaps: 


Author (X+2) Title. In proceedings of Conf'X...


The Chicago Manual of Style supports this style.