Dates of publication (9.4F & 1.4F; MARC 260 $c)
Most online monographs will have a single date of publication.
Common sources for date of publication for online monographs:
Date on PDF title page, verso, colophon, etc.
Publication date or copyright on Web page or home page (often at bottom)
Statement of date of publication on linked publisher or other page
Bibliographic record for print or other format version