Julian days and delta days with Date.prototype JavaScript extension

Date.prototype.julianDays() and Date.prototype.deltaDays()

/*  (c) APV, Artistic License 2, retain this notice  */

Date.prototype.julianDays = function () {
  if ( this.jd ) return this.jd;
  var y = this.getFullYear();
  var m = this.getMonth() + 1;
  var d = this.getDate();

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

  if ( m < 3 ) {
    m += 12;
    y--;
  }

  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() );
}

Sample usage

(JavaScript error)

<blockquote id="days_left">
  (JavaScript error)
</blockquote>

<script type="text/javascript">//<![CDATA[
var now = new Date();
var last_day_of_year = new Date(now.getFullYear(), 11, 31);

var delta = last_day_of_year.deltaDays(now);
var plural = delta == 1 ? 0 : 1;

var note = document.createTextNode("There " +
               ( plural ? "are " : "is " ) +
               delta + " day" +
               ( plural ? 's' : '' ) +
               " left in the year."
               );

var bq = document.getElementById("days_left");
bq.removeChild(bq.firstChild);
bq.appendChild(note);
//]]> </script>