Document.lastModified

The lastModified property of the Document interface returns a string containing the date and time on which the current document was last modified.

var string = document.lastModified;

This example alerts the value of lastModified.

alert(document.lastModified);
// returns: Tuesday, December 16, 2017 11:09:42

This example transforms lastModified into a Date object.

let oLastModif = new Date(document.lastModified);

This example transforms lastModified into the number of milliseconds since January 1, 1970, 00:00:00, local time.

let nLastModif = Date.parse(document.lastModified);

Note that as a string, lastModified cannot easily be used for comparing the modification dates of documents. Here is a possible example of how to show an alert message when the page changes (see also: JavaScript cookies API):

if (Date.parse(document.lastModified) > parseFloat(document.cookie.replace(/(?:(?:^|.*;)\s*last_modif\s*\=\s*([^;]*).*$)|^.*$/, "$1") || "0")) {
  document.cookie = "last_modif=" + Date.now() + "; expires=Fri, 31 Dec 9999 23:59:59 GMT; path=" + location.pathname;
  alert("This page has changed!");
}

…the same example, but skipping the first visit:

var
  nLastVisit = parseFloat(document.cookie.replace(/(?:(?:^|.*;)\s*last_modif\s*\=\s*([^;]*).*$)|^.*$/, "$1")),
  nLastModif = Date.parse(document.lastModified);

if (isNaN(nLastVisit) || nLastModif > nLastVisit) {
  document.cookie = "last_modif=" + Date.now() + "; expires=Fri, 31 Dec 9999 23:59:59 GMT; path=" + location.pathname;

  if (isFinite(nLastVisit)) {
    alert("This page has been changed!");
  }
}

If you want to know whether an external page has changed, please read this paragraph about the XMLHttpRequest() API.