code: function(year, month, day) {
          var date = this._validate(year, month, day,
              _exports.local.invalidDate || _exports.regionalOptions[''].invalidDate);
          year = date.year();
          month = date.month();
          day = date.day();
          if (year < 0) { year++; } // No year zero
          // Jean Meeus algorithm, "Astronomical Algorithms", 1991
          if (month < 3) {
              month += 12;
              year--;
          }
          var a = Math.floor(year / 100);
          var b = 2 - a + Math.floor(a / 4);
          return Math.floor(365.25 * (year + 4716)) +
              Math.floor(30.6001 * (month + 1)) + day + b - 1524.5;
      }
docs: Retrieve the Julian date equivalent for this date,
  i.e. days since January 1, 4713 BCE Greenwich noon.
  @memberof GregorianCalendar
  @param year {CDate|number} The date to convert or the year to convert.
  @param [month] {number} The month to convert.
  @param [day] {number} The day to convert.
  @return {number} The equivalent Julian date.
  @throws Error if an invalid date or a different calendar used.