What year do you give for results, arxiv or journal year? When giving a talk, if something is past ~1990, there's only the journal publication year to give to the audience. When something has only an arxiv year, then that's clearly the appropriate year. However, sometimes things take a long time to get published (refereeing and/or backlogs) and end up making things become anachronistic. One can now have a result that was posted in 2014, then published in 2016, while another paper is posted and published in 2015 that generalizes the paper in 2014. 

How do you give coherent and consistent choice of dates in a talk that are clear to the audience? Is there a guideline of which year to cite? 

(I'm in mathematics if that is relevant.)
I think you are trying to do two different things in one:


Give references: This should help others to find relevant literature and in fact, during talks one often uses just "Author Name(s), Year" to do so since this information often allows to track down the reference (at least in mathematics where people do not publish too many papers per year). If you want to do this, you should show the year of publication (you could also explicitly refer to the preprint version, but would need additional space…).
Give historical background: You may want to give the audience a feel for how things developed. My two cents here: I usually do not enjoy the talks that do this very much - I go to talks to learn new mathematics and "historical background" or anecdotes distract me (and also it seems hard to deliver them good). One exception is plenaries of people who are in the business for decades and really have something interesting  to say. Back to the point: If you want to give historical background you may need more space and should say "The result, obtained between 2013 and 2015 by A and published in three papers in 2015 and 2016…"


As a consequence, I think that this is a case in which consistency and clearness are not the same thing…