Blinckhorn N, in press. Molecular phylogeny of the genus Planta (Plantaceae). Some Botanical Journal.and then you had the time after print, when it would change to
Blinckhorn N, 2002. Molecular phylogeny of the genus Planta (Plantaceae). Some Botanical Journal 19: 34-41.(That or very similar is how we write reference lists in my area. It might be different in yours but the principle is the same everywhere.)
Now, however, papers actually have two dates of publication, once as online first or early view when they appear as PDFs on the journal's website and then again when they are finally printed, and the problem is really what to do when the two publication dates are in different years. In the above case, the researcher might want to update their staff website or publication list, or a colleague could decide to cite their paper, once it is available online. They would then write something like this:
Blinckhorn N, 2012. Molecular phylogeny of the genus Planta (Plantaceae). Some Botanical Journal. DOI: 00.0000/sbj.0000It was published in 2012, you can cite it, and the Digital Object Identifier (DOI) allows you to find it online. But then a few months later, the paper comes out in print. It now occupies page numbers in a certain issue of the journal and if somebody wants to cite it in it is suddenly
Blinckhorn N, 2013. Molecular phylogeny of the genus Planta (Plantaceae). Some Botanical Journal 30: 107-114.So, what is it, 2012 or 2013? Will that cause confusion? Speaking for myself, I think it already has caused me some in a few cases. And consider the not infrequent cases where a funding agency or employer wants a scientist to submit a list of their publications "in the last five years". How about counting the same paper as published (online first) in 2012 when asked for such a list in 2012 and as published (in print) in 2013 when asked for such a list in 2018, whatever serves you best at any given moment?
As more journals move to online only, this will become less of an issue because they will only have one publication date again.