Javascript Date format(pattern, gmt)

Date.prototype.format = function(pattern, gmt) {
 var result = '';
 var date = this;
 if(gmt != undefined) {
  var utc = date.getTime() + (date.getTimezoneOffset() * 60000);
  date = new Date(utc + (3600000 * gmt));
 }
 for (var i = 0; i < pattern.length; i++) {
  var c = pattern[i];
  switch (c) {//from www  .j a  v a 2  s .  c o  m
   case 'd':
    result += ('0' + date.getDate()).slice(-2);
    break;
   
   case 'm':
    result += ('0' + String(date.getMonth()+1)).slice(-2);
    break;
   
   case 'Y':
    result += date.getFullYear();
    break;
   
   case 'H':
    result += ('0' + date.getHours()).slice(-2);
    break;

   case 'i':
    result += ('0' + date.getMinutes()).slice(-2);
    break;

   case 's':
    result += ('0' + date.getSeconds()).slice(-2);
    break;

   case '\\':
    result += pattern[++i];
    break;

   default:
    result += c;
    break;
  }
 }
 return result;
}