The web can be used for research. However, many web pages do not have date created/last modified information on them. How would I get that date?
If they didn't include that, you're probably out of luck. You could look at the page source to see if it is embedded in there somewhere. But if it's not there, it's not there.
A great many web pages are dynamic, they are created at the time the page is requested by the browser, therefore the date created/modified changes with each refresh.