System.Globalization.PersianCalendar.ToDateTime C# (CSharp) Method

        public override DateTime ToDateTime(int year, int month, int day, int hour, int minute, int second, int millisecond, int era)  {
            // The year/month/era checking is done in GetDaysInMonth().
            int daysInMonth = GetDaysInMonth(year, month, era);
            if (day < 1 || day > daysInMonth) {
                BCLDebug.Log("year = " + year + ", month = " + month + ", day = " + day);
                throw new ArgumentOutOfRangeException(
                            "day",
                            String.Format(
                                CultureInfo.CurrentCulture,
                                Environment.GetResourceString("ArgumentOutOfRange_Day"),
                                daysInMonth,
                                month));
            }

            long lDate = GetAbsoluteDatePersian(year, month, day);

            if (lDate >= 0) {
                return (new DateTime(lDate * GregorianCalendar.TicksPerDay + TimeToTicks(hour, minute, second, millisecond)));
            } else {
                throw new ArgumentOutOfRangeException(null, Environment.GetResourceString("ArgumentOutOfRange_BadYearMonthDay"));
            }
        }

        public static int date_diff_persian(string d1, string d2)
        {
            string y = null;
            string m = null;
            string d = null;
            string r = null;

            System.Globalization.PersianCalendar g = new System.Globalization.PersianCalendar();
            DateTime in_date1 = default(DateTime);
            DateTime in_date2 = default(DateTime);

            try
            {
                y = d1.Substring(0, 4);          //جدا سازی 4 رقم عدد سال
                m = d1.Substring(5, 2);          //جدا سازی دو رقم عدد ماه
                d = d1.Substring(d1.Length - 2); //جدا سازی 2 رقم عدد روز
                                                 //-------------------------------------------------
                in_date1 = g.ToDateTime(Convert.ToInt32(NumericHelper.Val(y)), Convert.ToInt32(NumericHelper.Val(m)), Convert.ToInt32(NumericHelper.Val(d)), 0, 0, 0, 0);
                //**********************
                y = d2.Substring(0, 4);          //جدا سازی 4 رقم عدد سال
                m = d2.Substring(5, 2);          //جدا سازی دو رقم عدد ماه
                d = d2.Substring(d2.Length - 2); //جدا سازی 2 رقم عدد روز
                                                 //-------------------------------------------------
                in_date2 = g.ToDateTime(Convert.ToInt32(NumericHelper.Val(y)), Convert.ToInt32(NumericHelper.Val(m)), Convert.ToInt32(NumericHelper.Val(d)), 0, 0, 0, 0);
                //***********************
                r = DateHelper.DateDiff(DateHelper.DateInterval.Day, in_date1, in_date2).ToString();
            }
            catch (Exception ex)
            {
                r = "-1000000";
            }
            return(Convert.ToInt32(r));
        }