Javascript Date toISOTZString(tz)

Date.prototype.toISOTZString = function (tz) {
 // tz is as per Date.prototype.getTimezoneOffset, ie. minutes from UTC
 if (typeof tz === "undefined") {
  tz = this.getTimezoneOffset();//from w w w . j a  v  a  2s  .  c  o  m
 }
 var localDate = new Date(this - tz * 60 * 1000);
 var s = localDate.toISOString();
 s = s.substring(0, s.length-1);  // chop the trailing "Z"
 if (tz <= 0) {
  var dir = "+";
  tz *= -1;
 } else {
  var dir = "-";
 }
 var hrs = 0;
 var rem = tz;
 while (rem >= 60) {
  hrs++;
  rem -= 60;
 }
 var mins = rem;
 if (hrs < 10) {
  hrs = "0" + hrs;
 }
 if (mins < 10) {
  mins = "0" + mins;
 }
 return s + dir + hrs + mins;

};