_monthNames = new Array('January','February','March','April','May','June','July','August','September','October','November','December');
_dayNames = new Array('Sunday','Monday','Tuesday','Wednesday','Thursday','Friday','Saturday');

// extend existing objects
String.prototype.zf = function(l) { return '0'.string(l - this.length) + this; }
String.prototype.string = function(l) { var s = '', i = 0; while (i++ < l) { s += this; } return s; }
String.prototype.trim = function () { return this.replace(/^\s*(\S*(\s+\S+)*)\s*$/, '$1');};
Number.prototype.zf = function(l) { return this.toString().zf(l); }

Date.prototype.format = function(f) {
  if (!this.valueOf()) return 'n.a.';
  
  var d = this;
  return f.replace(/(yyyy|yy|y|MMMM|MMM|MM|M|dddd|ddd|dd|d|HH|H|hh|h|mm|m|ss|s|t|x)/gi,
    function($1) {
      switch ($1) {
        case 'yyyy': return d.getFullYear();
        case 'yy':   return (d.getFullYear()%100).zf(2);
        case 'y':    return (d.getFullYear()%100);
        case 'MMMM': return _monthNames[d.getMonth()];
        case 'MMM':  return _monthNames[d.getMonth()].substr(0, 3);
        case 'MM':   return (d.getMonth() + 1).zf(2);
        case 'M':    return (d.getMonth() + 1);
        case 'dddd': return _dayNames[d.getDay()];
        case 'ddd':  return _dayNames[d.getDay()].substr(0, 3);
        case 'dd':   return d.getDate().zf(2);
        case 'd':    return d.getDate();
        case 'HH':   return d.getHours().zf(2);
        case 'H':    return d.getHours();
        case 'hh':   return ((h = d.getHours() % 12) ? h : 12).zf(2);
        case 'h':    return ((h = d.getHours() % 12) ? h : 12);
        case 'mm':   return d.getMinutes().zf(2);
        case 'm':    return d.getMinutes();
        case 'ss':   return d.getSeconds().zf(2);
        case 's':    return d.getSeconds();
        case 't':    return d.getHours() < 12 ? 'am' : 'pm';
        case 'x': var i = d.getDate() % 10; return 'thstndrd'.substr(2 * (i < 4) * i, 2);            
      }
    }
  );
}