Some websites contain statement such as "This website was last revised
on <date>." However, there are lots of websites that contain no such
statement. Is there some way to find out the last revision date,
either approximately or exactly, for a website that doesn't provide
this information explicitly?

I know about http://www.archive.org and use it a lot. For old,
archived websites, the revision date is provided in www.archive.org,
but I'd like to get this kind of information for a current website not
listed in www.archive.org. I'd like to be able to answer the question:
Am I looking at a website that was last revised several months ago, or
was it revised recently -- and in either case, when was it last
revised?

quester2,

The HTTP 1.1 protocol, in use by the vast majority of web servers to
communicate data from the server to the web client, includes an
information field called "Last-Modified" with each request to the
server.

http://www.httpsniffer.com/http/1429.htm

Web servers send this information in the "header" of a request
(information that is used by your browser to display the content of
the web page) but is not actually displayed as content.  There are
several tools available for viewing the header information of a web
page, but the one I use most often is Rex Swain's HTTP viewer:

http://www.rexswain.com/httpview.html

On this page, enter the name of the webpage you are looking for in the
URL section.  For "Request-Type" change the default selection to
"HEAD", leave all other options as they are, and click the Submit
button.  If the "Last-Modified" header information is included, the
last updated date will appear on the resulting page.

For example, for the URL http://www.cnn.com , the following header
information is retrieved:

Header (Length = 295):
HTTP/1.1?200?OK(CR)
(LF)
Date:?Wed,?17?Nov?2004?06:46:32?GMT(CR)
(LF)
Server:?Apache(CR)
(LF)
Content-Type:?text/html(CR)
(LF)
Last-Modified:?Wed,?17?Nov?2004?06:46:29?GMT(CR)
(LF)
Cache-Control:?max-age=60,?private(CR)
(LF)
Vary:?Accept-Encoding,User-Agent(CR)
(LF)
Expires:?Wed,?17?Nov?2004?06:47:29?GMT(CR)
(LF)
Content-Length:?59164(CR)
(LF)
Connection:?close(CR)
(LF)
(CR)
(LF)

Ignoring the other information that we are not concerned with, the
"Last-Modified" field shows the last modified date in GMT standard
time as November 17th, 2004 6:47am for the CNN website.

Unfortunately, although including this information is encouraged, the
"Last-Modified" field is optional, meaning that there will be web
servers that will not return this information.  If the "Last-Modified"
field information is not given by the web server, then this
information is not provided by the server, and it is impossible to
know the specific time and date that any file was updated.

Sites Referenced:
-----------------
HTTP Look
http://www.httpsniffer.com/

Rex Swain's Web Site
http://www.rexswain.com/

Google Search Terms Used:
-------------------------
"HTTP 1.1" "Last-Modified"

---

Clarification of Answer by hailstorm-ga on 17 Nov 2004 20:37 PST

quester2,

Thank you for the kind comments, rating, and tip! (in no particular order)

One clarification I would like to make is that the availability of the
"Last-Modified" information is not set by the people in charge of the
content, but by the people in charge of the web servers that deliver
the content to your browser.  These tasks are not necessarily handled
by the same people, especially for large sites such as the CNN website
that I used for my example.

With regards to your followup question, are you referring to general
knowledge of HTTP protocol, or information on websites and the World
Wide Web in general?

Hello, hailstorm-ga. Superb, clear, complete answer. And the answer
includes the fact that the "Last-Modified" information is sometimes
not entered by the creator of a website and is therefore impossible to
know.

Suppose I want to increase my general knowledge of things like this.
Where is a good place to start? Can you suggest some reading material
or websites that have this kind of information?

If you go to http://www.archive.org/web/web.php and then type the name
of the site you want to check, it'll give you when the website was
updated.

Not sure on the accuracy, but it'll give you an idea.

Hope this was helpful!