var monthNames = new Array( "January",
  "February", "March", "April", "May", "June", "July", "August",
  "September", "October", "November", "December");

function customDateString( aDate ) {
  var theDay = aDate.getDate();
  var theMonth = monthNames[ aDate.getMonth() ];
  var theYear = aDate.getFullYear();
  return theMonth + " " + theDay + ", " + theYear; }

lastChange = new Date( document.lastModified );
document.write( customDateString( lastChange )  );