Sometimes, people using an author-date style want to mention the author directly in a sentence rather than in the in-text citation itself. For example, "As argued by Foucault (2008)..." rather than (Foucault, 2008). It is important to use the Zotero toolbar for this type of citation so that Zotero recognises it as a citation and adds it to the bibliography. In this case, the citation should be inserted into the text using the "classic view" option (by clicking on the small triangle next to the Z). Then simply tick the "suppress author" box on the right. You will then have an in-text citation with only the year.