You need to bind the DropDownList this way if you are using SqlDataReader.
Namespaces
using System.Data;
using System.Data.SqlClient;
using System.Configuration;
 C#
protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        PopulateDropDownLIst();
    }
}
public void PopulateDropDownLIst()
{
    ddlRepDate.Items.Clear();
    ddlRepDate.Items.Add(new ListItem("--Select Reporting Date--", ""));
    ddlRepDate.AppendDataBoundItems = true;
    using (SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["constr"].ConnectionString))
    {
        string strQuery = "SELECT OrderDate,RIGHT ('0' + RTRIM(DAY(OrderDate)), 2) + '-' + RIGHT ('0' + RTRIM(MONTH(OrderDate)), 2) +'-' + RTRIM(YEAR(OrderDate)) AS mydate from Orders ";
        using (SqlCommand cmd = new SqlCommand(strQuery, conn))
        {
            conn.Open();
            SqlDataReader reader = cmd.ExecuteReader();
            ddlRepDate.DataTextField = "mydate";
            ddlRepDate.DataValueField = "OrderDate";
            ddlRepDate.DataSource = reader;
            ddlRepDate.DataBind();
            conn.Close();
        }
    }
}