Detect last-modified HTTP headers (via ajax) and use them

lastmodified.js

function updated_when() {
	var xmlhttp;
	var last_modified = {};
	if (window.XMLHttpRequest) {  xmlhttp=new XMLHttpRequest(); }
	else  { xmlhttp=new ActiveXObject("Microsoft.XMLHTTP"); }
	xmlhttp.onreadystatechange=function()  {
	  if (xmlhttp.readyState==4 && xmlhttp.status==200) {
	    var lm_str = xmlhttp.getResponseHeader('Last-Modified');
		    if (lm_str != '') {
			    last_modified.date = new Date(lm_str);
			    var date_div = document.createElement("div");
			    date_div.innerHTML = "this page was edited on "+ (last_modified.date.getMonth()+1) + "/" + last_modified.date.getDate();
				last_modified.html = date_div;
				return last_modified;
		    }
	    }
	  }
	xmlhttp.open("GET",location.href,true);
	xmlhttp.send();
}