Return UTC as a floating point Julian date.

def _utc_float(self):
        """Return UTC as a floating point Julian date."""
        tai = self.tai
        leap_dates = self.ts.leap_dates
        leap_offsets = self.ts.leap_offsets
        leap_reverse_dates = leap_dates + leap_offsets / DAY_S
        i = searchsorted(leap_reverse_dates, tai, 'right')
        return tai - leap_offsets[i] / DAY_S