I just tried your code using a DB I had to hand:
SqlConnection con = new SqlConnection(@"Data Source=GRIFFPC\SqlExpress;Initial Catalog=VideoMaster;Integrated Security=True");
SqlDataAdapter da = new SqlDataAdapter("SELECT * FROM Videos", con);
DataTable dt = new DataTable();
DateTime dat = (DateTime)dt.Compute("MAX(InsertDate)", null);
DateTime dat2 = Convert.ToDateTime(dt.Compute("MAX(InsertDate)", null));
Both dates were correct and valid. So, what am I doing that is different from you?