dateFormat.js

Date.prototype.format = function(fstr, utc) {
  var that = this;
  utc = utc ? 'getUTC' : 'get';
  return fstr.replace (/%[YmdHMS]/g, function (m) {
    switch (m) {
    case '%Y': return that[utc + 'FullYear'] ();
    case '%m': m = 1 + that[utc + 'Month'] (); break;
    case '%d': m = that[utc + 'Date'] (); break;
    case '%H': m = that[utc + 'Hours'] (); break;
    case '%M': m = that[utc + 'Minutes'] (); break;
    case '%S': m = that[utc + 'Seconds'] (); break;
    default: return m.slice (1); 
    }    
    return ('0' + m).slice (-2);
  });
};

// demo
a = new Date();
console.log(a.format ("%Y-%m-%d %H:%M:%S", true) );
console.log(a.format ("%d/%m/%Y %H:%M:%S", true));
console.log(a.format ("%d/%m/%Y %H:%M:%S", false));
console.log(a.format ("%m", true) ); // formated month
console.log(a.format ("%d", true) ); // formated date