Citation, Documentation of Sources
Q. More and more journals publish articles first online before the print edition. In some cases, the online version is published one or more years before the print version. For author-date, do we include both dates (i.e.,  2021), or do we ignore the year of the online version?
A. For the citation, use the date of the numbered journal issue rather than the date the article was originally published online, unless the article has not yet been assigned to a numbered issue. For example, here’s a reference list entry and parenthetical citation for an article in Freshwater Science that was received by the journal on June 17, 2019, accepted for publication on December 4, 2020, and published ahead of print on April 16, 2021—but not yet assigned to an issue:
Savoy, Philip, Emily Bernhardt, Lily Kirk, Matthew J. Cohen, and James B. Heffernan. 2021. “A Seasonally Dynamic Model of Light at the Stream Surface.” Freshwater Science. Published ahead of print, April 16. https://doi.org/10.1086/714270.
(Savoy et al. 2021)
Here’s an article received by that same journal on December 8, 2019, accepted on August 3, 2020, published online on February 12, 2021, and assigned to a numbered issue for March 2021:
Vanlandingham, Amanda L., Richard H. Walker, Adam Alford, and Sally A. Entrekin. 2021. “Intermittency Mediates Macroinvertebrate and Crayfish Effects on Leaf Breakdown in Temperate Headwater Streams.” Freshwater Science 40, no. 1 (March): 21–38. https://doi.org/10.1086/713094.
(Vanlandingham et al. 2021)
Note that many journal publishers include the dates of receipt, acceptance, and online (or ahead-of-print) publication as part of the record for an article once it’s been assigned a place in a numbered issue. If any of those dates are relevant, they can be discussed or noted in the text. The citation, on the other hand, will be most useful if it points to the numbered issue. For preprints, which are cited separately, see CMOS 14.173.