A strftime for Prototype

Object.extend(Date.prototype, {
strftime: function(format) {
 var day = this.getUTCDay(), month = this.getUTCMonth();
 var hours = this.getUTCHours(), minutes = this.getUTCMinutes();
 function pad(num) { return num.toPaddedString(2); };

 return format.gsub(/\%([aAbBcdDHiImMpSwyY])/, function(part) {
   switch(part[1]) {
     case 'a': return $w("Sun Mon Tue Wed Thu Fri Sat")[day]; break;
     case 'A': return $w("Sunday Monday Tuesday Wednesday Thursday
                          Friday Saturday")[day]; break;
     case 'b': return $w("Jan Feb Mar Apr May Jun Jul Aug Sep Oct
                          Nov Dec")[month]; break;
     case 'B': return $w("January February March April May June July
                          August September October November December")
                      [month]; break;
     case 'c': return this.toString(); break;
     case 'd': return this.getUTCDate(); break;
     case 'D': return pad(this.getUTCDate()); break;
     case 'H': return pad(hours); break;
     case 'i': return (hours === 12 || hours === 0) ? 12 : 
                      (hours + 12) % 12; break;
     case 'I': return pad((hours === 12 || hours === 0) ? 12 :
                          (hours + 12) % 12); break;
     case 'm': return pad(month + 1); break;
     case 'M': return pad(minutes); break;
     case 'p': return hours > 11 ? 'PM' : 'AM'; break;
     case 'S': return pad(this.getUTCSeconds()); break;
     case 'w': return day; break;
     case 'y': return pad(this.getUTCFullYear() % 100); break;
     case 'Y': return this.getUTCFullYear().toString(); break;
   }
 }.bind(this));
}
});

http://alternateidea.com/blog/articles/2008/2/8/a-strftime-for-prototype