Transformer la date julienne

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Voir:

http://quasar.as.utexas.edu/BillInfo/JulianDatesG.html

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int PDate::toJulianDay() const
{
  if (year < 1582)
    PLERROR("toJulianDay works safely only for year > 1581 (%d)", year);
  int jy = (month>2) ? year : year-1;
  int jm = (month>2) ? month : month+12;
  int ja = (int)(jy/100);
  int jb = (int)(ja/4);
  int jc = 2 - ja + jb;
  int je = (int)(365.25*(jy + 4716));
  int jf = (int)(30.6001*(jm + 1));

  return jc + day + je + jf - 1524;
}

PDate::PDate(int julian_day)
{
  int jw = (int)((julian_day - 1867216.25)/36524.25);
  int jx = (int)(jw/4);
  int ja = julian_day + 1 + jw - jx;
  int jb = ja + 1524;
  int jc = (int)((jb - 122.1)/365.25);
  int jd = (int)(365.25*jc);
  int je = (int)((jb - jd)/30.6001);
  int jf = (int)(30.6001*je);

  day = jb - jd - jf;
  month = (je>13) ? je-13 : je-1;
  year = (month>2) ? jc-4716 : jc-4715;
}