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
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:
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):
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.
Rex Swain's Web Site
Google Search Terms Used:
"HTTP 1.1" "Last-Modified"
Clarification of Answer by
17 Nov 2004 20:37 PST
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?