In older times, sometimes a publication would be dated with one year on the title page but actually issued in a later year. Nowadays, taxonomic names can be valid from the online version of record but the article may be put into a volume dated in the next year. This isn't a huge problem because enterers can input the correct year (rather than the printed year), but there's a potential for minor confusion. Sometimes these citations are given as Author, 1920 (1921) or Author, 2020 (2019) - so it could be useful to capture both dates in a revised reference entry form.
In older times, sometimes a publication would be dated with one year on the title page but actually issued in a later year. Nowadays, taxonomic names can be valid from the online version of record but the article may be put into a volume dated in the next year. This isn't a huge problem because enterers can input the correct year (rather than the printed year), but there's a potential for minor confusion. Sometimes these citations are given as Author, 1920 (1921) or Author, 2020 (2019) - so it could be useful to capture both dates in a revised reference entry form.