31.1. The date of effective publication is the date on which the printed matter became available as defined in Art. 29 and 30. In the absence of proof establishing some other date, the one appearing in the printed matter must be accepted as correct.
Ex. 1. Individual parts of Willdenow's Species plantarum were published as follows: 1(1), Jun 1797; 1(2), Jul 1798; 2(1), Mar 1799; 2(2), Dec 1799; 3(1), 1800; 3(2), Nov 1802; 3(3), Apr-Dec 1803 (and later than Michaux's Flora boreali-americana); 4(1), 1805; 4(2), 1806; these dates, which are partly in disagreement with the years on the title-pages of the volumes, are presently accepted as the correct dates of effective publication.
Ex. 2. T. M. Fries, "Lichenes arctoi", was first published as an independently paginated preprint in 1860, which predates the identical version published in a journal (Nova Acta Reg. Soc. Sci. Upsal. 3(3): 103-398. 1861).
31.2. When separates from periodicals or other works placed on sale are issued in advance, the date on the separate is accepted as the date of effective publication unless there is evidence that it is erroneous.
Ex. 3. Publication in separates issued in advance: the
names of the Selaginella species published by Hieronymus
(in Hedwigia 51: 241-272) were effectively published on 15 October
1911, since the volume in which the paper appeared, though dated
1912, states (p. ii) that the separate appeared on that date.
31A.1. The date on which the publisher or his agent delivers
printed matter to one of the usual carriers for distribution to
the public should be accepted as its date of effective publication.