static public void ClearDateTime(out DateTime d1)
        {
            d1 = DateTime.MinValue;
            
            // same as below
            //return new DateTime();
            //return new DateTime(1, 1, 1, 0, 0, 0);
        }
        static public bool IsMinValue(DateTime d1)
        {
            return (d1 == DateTime.MinValue);
        }
        static public DateTime FromYMDHNS(int iY, int iM, int iD, int iH, int iN, int iS)
        {
            return new DateTime(iY, iM, iD, iH, iN, iS);
        }
        static public DateTime FromYMD(int iY, int iM, int iD)
        {
            return new DateTime(iY, iM, iD);
        }