void DatasimDate::jul_to_greg(julTy& day, julTy& month, julTy& year) const
/*
 * get the Gregorian DatasimDate (returned in the referenced variables
 * day, month and year) from the number of Julian days (member
 * Julian_days) from this DatasimDate. Algorithm 199 from Communications
 * af the ACM Volume6, No.8, (Aug. 1963) p.144. If the number of
 * Julian days = 0, return January 1, 1900. Gregorian calender started
 * September 14, 1752. 
 */
{
 if (Julian_days == 0) { // return January 1, 1900
 day = 1; month = 1; year = 1900;
 return;
 }
 else { // This is the algorithm.
 julTy j = Julian_days - 1721119l;
 year = (julTy)(((j << 2) - 1) / 146097l);
 j = (j << 2) - 1 - 146097l * year;
 day = (julTy)(j >> 2);
 j = ((day << 2) + 3) / 1461;
 day = (julTy)((day << 2) + 3 - 1461 * j);
 day = (day + 4) >> 2;
 month = (5 * day - 3) / 153;
 day = 5 * day - 3 - 153 * month;
 day = (day + 5) / 5;
 year = (julTy)(100 * year + j);
 if (month < 10)
 month += 3;
 else {
 month -=9;
 year++;
 }
 }
}