  With the acceleration of scholarly communication in the digital era, the
publication year is no longer a sufficient level of time aggregation for
bibliometric and social media indicators. Papers are increasingly cited before
they have been officially published in a journal issue and mentioned on Twitter
within days of online availability. In order to find a suitable proxy for the
day of online publication allowing for the computation of more accurate
benchmarks and fine-grained citation and social media event windows, various
dates are compared for a set of 58,896 papers published by Nature Publishing
Group, PLOS, Springer and Wiley-Blackwell in 2012. Dates include the online
date provided by the publishers, the month of the journal issue, the Web of
Science indexing date, the date of the first tweet mentioning the paper as well
as the Altmetric.com publication and first-seen dates. Comparing these dates,
the analysis reveals that large differences exist between publishers, leading
to the conclusion that more transparency and standardization is needed in the
reporting of publication dates. The date on which the fixed journal article
(Version of Record) is first made available on the publisher's website is
proposed as a consistent definition of the online date.
