Get global relative timestamp in Javascript

timestamp.js

/* 
Note : Always make sure that input given to instance of Date class is in UTC. If Date is in UTC then only
proper relative timestamp can be calculated. 

By simply calling getRelativeTimestamp on any date object will convert it to relative timestamp
*/

Date.prototype.getRelativeTimestamp = function () {
  var tzoffset = this.getTimezoneOffset() * 60 * 1000; // Calculate timezone offset
  var currDate = new Date();
  var relativeDate = new Date(this.getTime() - tzoffset); // Input date will be converted to relative time
  var delta = Math.round((currDate - relativeDate)/1000);
  var rtstamp = '';

  if(delta < 60) { rtstamp = delta + ' seconds ago'; }
  else if(delta < (60*60)) {
    rtstamp = Math.round(delta/60) + ' minutes ago';
  }
  else if(delta < (60*60*24)) {
    rtstamp = Math.round(delta/(60*60)) + ' hours ago';
  }
  else if(delta < (60*60*24*31)) {
    rtstamp = Math.round(delta/(60*60*24)) + ' days ago';
  } else {
    rtstamp = relativeDate.toDateString();
  }

  return rtstamp;
};