Many journals, and most abstracting and indexing services (including Medline and Institute for Scientific Information) still cite the print date as the publication date.
This is an evolving standard, however, and the trend now is for publishers to state both the 'Online Publication Date' and the 'Print Publication Date'.
Nature Portfolio will continue to publish both dates for our own papers, and we hope that the scientific communities and abstracting and indexing services will recognize these dates.
For the time being, the reference lists of our papers will continue to follow the standard convention of citing by print publication date. Thus, there will be no immediate change to the reference format instructions in our Guides to Authors (but see below for how to cite papers that have not yet appeared in print).
We expect, however, to review this policy regularly as community standards evolve.
For legal purposes (for example, establishing intellectual property rights), it is our assumption that online publication will constitute public disclosure.
This is, however, for the courts to decide; our role as a publisher is to provide clear documentation of the publication history, online and in print