problem with conditional operator ?

It seems like Txt_reference_date.Text is not recognized as a valid DateTime. Conditional operator is not the issue here.

You can try :

DateTime d;
obenitiypersonal.reference_date =
   (DateTime.TryParse(Txt_reference_date.Text, out d))
   ? d
   : DateTime.MinValue;

Here I assume that obenitiypersonal.reference_date is a DateTime struct.

If obenitiypersonal.reference_date were a DateTime? (Nullable<datetime></datetime>), you could write :

DateTime d;
obenitiypersonal.reference_date =
   (DateTime.TryParse(Txt_reference_date.Text, out d))
   ? d
   : null;

Basically you cannot get a valid DateTime value out of DBNull.Value, like you do.