Rule 17.12 in The Chicago Manual of Style:
Access dates in online source citations are of limited value, since previous versions will often be unavailable to readers (not to mention that an author may have consulted several revisions across any number of days in the course of research). Chicago therefore does not generally recommend including them in a published citation. For sources likely to have substantive updates, however, or in time-sensitive fields such as medicine or law where even small corrections may be significant, the date of the author’s last visit to the site may usefully be added.
I disagree. Online references always should include the date last accessed:
- Websites like The Internet Archive make it possible to access earlier iterations of a webpage, sorted by date.
- Including the date last accessed reminds readers that if they review the author’s references, the websites may not contain the same content as when the author visited it.
Authors can help subsequent researchers by saving every webpage they use and not just bookmarking them (all Internet browsers allow users to save a copy of a website). Hence, if the website has changed, all a researcher needs to do is contact a paper’s author and ask for a copy of the archived website to view the source as the author saw it.