Convert Datetime Array to float64 ndarray of Julian Dates.
        0 Julian date is noon January 1, 4713 BC.
        http://en.wikipedia.org/wiki/Julian_day

def to_julian_date(self):
        """
        Convert Datetime Array to float64 ndarray of Julian Dates.
        0 Julian date is noon January 1, 4713 BC.
        http://en.wikipedia.org/wiki/Julian_day
        """

        # http://mysite.verizon.net/aesir_research/date/jdalg2.htm
        year = np.asarray(self.year)
        month = np.asarray(self.month)
        day = np.asarray(self.day)
        testarr = month < 3
        year[testarr] -= 1
        month[testarr] += 12
        return (day +
                np.fix((153 * month - 457) / 5) +
                365 * year +
                np.floor(year / 4) -
                np.floor(year / 100) +
                np.floor(year / 400) +
                1721118.5 +
                (self.hour +
                 self.minute / 60.0 +
                 self.second / 3600.0 +
                 self.microsecond / 3600.0 / 1e+6 +
                 self.nanosecond / 3600.0 / 1e+9
                 ) / 24.0)