// datepart: 'y', 'm', 'w', 'd', 'h', 'n', 's'
Date.dateDiff = function(datepart, fromdate, todate) {
  datepart = datepart.toLowerCase();
  var diff = todate - fromdate;
  var divideBy = { w:604800000,
                   d:86400000,
                   h:3600000,
                   n:60000,
                   s:1000 };
  console.log("Diff " +diff);
  return Math.floor( diff/divideBy[datepart]);
}
//Set the two dates
var y2k  = new Date(2018, 8, 1);
var today= new Date(2018, 8, 20);
console.log('Weeks since the new millenium: ' + Date.dateDiff('d', y2k, today)); //displays 19