Sometimes, people using an author-date style wish to mention the author directly into a sentence rather than in the in-text citation itself. For instance, "As argued by Foucault (2008)..." rather than (Foucault, 2008). It is essential to use the Zotero toolbar for this kind of citation, in order that Zotero recognizes it as a citation and add it to the bibliography. The citation should, in this case, be inserted in the text using the "classic view" option (by clicking on the little triangle beside the Z). Then just check the "suppress author" box on the right. You will have an in-text citation with only the year.