retrieving a datetime value from sql using the select statement

The problem is most likely in the WHERE clause of your Sql statement. Incidentally, your Sql is also rip for Sql Injection attacks. Instead of doing string manipulation, it is a best practice to use SqlParameters. Try the following instead and let me know how you get on.

string str = "select Sign_in, Date from Attendance where Eno= @Eno and Date = @Date";
var cmd = new SqlCommand(str, conn);
cmd.Parameters.AddWithValue("Eno", b);
cmd.Parameters.AddWithValue("Date", dateTimePicker1.Value.Date);
var dr = cmd.ExecuteReader();

Cheers,

Rob