[GuiA]

Papers are published in journal/conference proceedings/etc. that will have the date of the issue ("Transactions for the International Symposium on Computational Yak Shaving 2018"). The paper might have been written in 2017, but published in 2018, which means that when it gets cited it will be as "ABC et al., 2018".

Papers without a date are usually preprints, or published independently (e.g. on the author's website) while expecting actual publication at some point.