Extending the native Date() class

You can see this class in use in Using JS code libraries, part 2.

/*
  Code from "Developing Featherweight Web Services with JavaScript"
      http://feather.elektrum.org/
  (c)An Elektrum Press, retain this notice
      License: http://feather.elektrum.org/appendix/licenses.html
*/

Date.prototype.julianDays = function () {
  if ( this.jd ) return this.jd;

  var y = this.getFullYear();
  var m = this.getMonth() + 1;
  var d = this.getDate();
  // this is a fairly well know algorithm for converting 
  if ( m < 3 ) {
    m += 12;
    y--;
  }

// Julian Days conversion algorithm is from "Astronomical Algorithms,"
// 1991, page 61, Jean Meeus. Found via Peter Baum.

  this.jd = 
    Math.floor( 365.25*( y + 4716 ) ) + 
    Math.floor( 30.6001*( m + 1 ) ) + d + 2 -
    Math.floor( y/100 ) +
    Math.floor( Math.floor( y/100 ) / 4 ) - 1524.5;

  return this.jd;
}

Date.prototype.deltaDays = function ( date2 ) {
  // might want to try/throw custom errors here
  return Math.abs( this.julianDays() - date2.julianDays() );
}

Date.namesOfMonths = new Array( 'January' ,'February'
                                ,'March' ,'April' ,'May'
                                ,'June' ,'July','August'
                                ,'September' ,'October'
                                ,'November' ,'December' );

Date.prototype.getMonthName = function( monthIndex ) {
  if ( ! ( monthIndex && typeof( monthIndex ) == 'number' &&
           monthIndex >= 0 && monthIndex < 12
          ) )
    monthIndex = this.getMonth();

  return Date.namesOfMonths[monthIndex];
};

Live tests

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