Important function missing from source code.

Important function missing from source code.

Was just going through the source of code datetimepicker HERE  . the source code can be seen HERE  . now there is this function 

  2.         Date.prototype.dateFormat = function( format ){
  3.           //you code for convert date object to format string
  4.           //for example
  5.           switch( format ){
  6.             case 'd': return this.getDate();
  7.             case 'H:i:s': return this.getHours()+':'+this.getMinutes()+':'+this.getSeconds();
  8.             /*case 'h:i a': return ((this.getHours() %12) ? this.getHours() % 12 : 12)+':'+this.getMinutes()+(this.getHours() < 12 ? 'am' : 'pm');*/
  9.           }
  10.         }
That gets used in the plugin source line 15+ times but is not defined in the source at all , if you check the demo page it is defined in the HTML , is this an error by the plugin author , i know this is probably not the right place to ask , but still . 

if i just import the plugin , and initialize the plugin like so:

  1.         <input id="datetimepicker" type="text">

  1. $(function(){

  3.                     $('#datetimepicker').datetimepicker({
  4.                       format:'d.m.Y H:i',
  5.                       inline:true,
  6.                       lang:'ru'
  7.                     });
  8.           });
I get the following error.
  1. Uncaught TypeError: start.dateFormat is not a function
unless i add this in my plugin code too

  1.   Date.prototype.dateFormat = function( format ){
  2.   //you code for convert date object to format string
  3.   //for example
  4.   switch( format ){
  5.     case 'd': return this.getDate();
  6.     case 'H:i:s': return this.getHours()+':'+this.getMinutes()+':'+this.getSeconds();
  7.     /*case 'h:i a': return ((this.getHours() %12) ? this.getHours() % 12 : 12)+':'+this.getMinutes()+(this.getHours() &lt; 12 ? 'am' : 'pm');*/
  8.   }
  9. }
My question is why is the above code not included in the source code ?