Reference a web page by author and year: which date to use in citation; publication date or revision date?
I searched for your question and found these content which may help you. I found that, in citation of web pages or online resources, it is better (not a must) to cite with the updated date and the date when the reader has accessed the online content.

In this web pages about APA, Chigago and CSE styles, under the update date it is written:


  This date is when the page was last changed. The last update date of a
  web page is usually given at the bottom or top of a page.  A posting
  date may also be given on the page one level higher (such as a page
  that is an index of articles).


Also, as a recommendation I read the following content on this webpage:


  Date of publication or date of last revision
  
  
  The date a Web document was created or last updated is frequently listed at the bottom.
  If a document includes both a date of creation and a date it was last updated, use only the latter.
  


It seems that as there is no general rule about the date included in online resources, the writer preferred the revised date.

However, I also checked the following book 

The Chicago Manual of Style, sixteenth edition; essential guide for writers, editors and publishers; ebook, The University of Chicago press,
2010.

and in this section on the 1700th page of the PDF file, 

14: DOCUMENTATIONI: NOTES AND BIBLIOGRAPHY Chapter Contents &gt;&gt;  
    Source Citations: An Overview &gt;&gt;  Considerations for Electronic Sources


the following guide is presented; as we see in the index of the document, this guide applies to websites and web pages:


  14.8 "LAST MODIFIED" AND OTHER REVISION DATES  
  
  Some electronic documents will include a date on each page or screen indicating the last time the document was modified or revised. There are no accepted standards for this practice, and for formally published material the date of publication is generally more important. A revision date should be included, however, if it is presented as the de facto date of publication or is otherwise the only available date. Such dates may be particularly useful for citing wikis and other continuously updated works.


On the contrary it seems that in the APA style, the preferred date is the update date rather than the publication date in the previous quoted text. In the apastyle website under a blog content about The Generic Reference: When? we read:


  Online Documents
  
  Online material can be tricky to date properly. If the date is not
  apparent at the beginning of the document you’re citing, look at the
  end (e.g., APA Guidelines for Providers of Psychological Services to Ethnic, Linguistic, and Culturally Diverse Populations was finalized
  in 1990, so that’s the date to use).
  
  But look out for a footer that says, “This page was last modified on
  [date].” This is not the date to use! It could be the date that the
  document was published, but it’s more likely to be the date it was put
  online or the date when the webmaster added code for a dancing Freud
  to the page.
  
  Some sites place a copyright date for the website at the bottom of
  every page. Check a few pages on the site; if the identical statement
  appears on every page, it’s a site-wide footer, so that’s not the date
  you’re looking for either. (See "Zip, Zero, Zilch," below, for the
  best solution.)


As a conclusion; I found that if you want to cite an online content, pay attention into two dates; the date when you visited the online content and the date the content is created. If you have the date when the content is edited and updated, use this newer date. However, I recommend you to read the instructions of the style which you are using, they may have specific recommendations and guides on this issue. Also, be aware, it happens sometimes that the online content is written on an older date when the website update date is globally mentioned in the footer of the website's pages, you should use the date the page you are referring to is created or edited, not the date all the website is updated.