DateTime d = new DateTime(40010); //invalid

DateTime myDate = new DateTime(1900, 1, 1).AddDays(days);

private DateTime AS400 = new DateTime(1900, 1, 1);

...


DateTime myClrDT = AS400.AddDays(days);

DateTime d = Convert.ToDateTime(reader["DateField"]);

SELECT
   CAST(0 AS datetime) AS dateTimeZero,
   CAST(0 AS smalldatetime) AS smallDateTimeZero

dateTimeZero              smallDateTimeZero
=======================   ===================
1900-01-01 00:00:00.000   1900-01-01 00:00:00

select date(15+719892) as date1 from library1.file1