getResponseHeader('last-modified'); does not change value

Question

var page = 'data/appointments/<? echo $_SESSION['name']; ?><? echo $_SESSION['last']; ?>App.html';
var lM;
function checkModified(){
   $.get(page, function(a,a,x){
      var mod = x.getResponseHeader('last-modified');
      alert (lM);
      alert ("page" +mod);
});
}

when i alert the last-modified from my page, it outputs the same value, even when when i deleted all my cookies and cache , deleted the file from the server and replace it.

it still outputs one value

Tue , Oct 23, 2012 3:37:41 GMT