Conference held in year X, proceedings published in X+2 - which to use?
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.