My impression is that the year of issue is based on two factors:
- The date marketed by the company; and
- The date of release.
As an (almost?) absolute rule, if the product is released between December of the previous year (say 2016) and January of the following year (say 2018), and is marketed as being a product dated of the year that is the 12 months in between (say 2017), then it's clearly dated the year when it was marketed (so, 2017). That's 99.9% of all issues.
Occasionally, a product is marketed as being of a year outside of the above parameters. An example is the Topps Debut sets of 1989-91 (released early in the following year in each case, I think in March). This site and Beckett use the year of issue, although COMC uses both years (i.e., 1989-90, 1990-91 and 1991-92). Another example is "2011" Panini Limited, which was released in May(!) 2012.
If it were me, I would designate those sets in the way COMC does (year of issue plus a dash and year of release). But it's not me.
In any event, copyright date is irrelevant.
I'd be eternally grateful to anyone who can help me find a 1998 Bryan Braves (non-perforated) Kerry Ligtenberg and a 2000 Nabisco Albertson's Astros Chris Truby.
Last missing items in a 10,000-card "set."