Increment Value From Dropdownlist

Here i have set the values in DropDownList From Jan 2013 to December just for understanding purpose but you can bind it from Data base.

<form id="form1" runat="server">
    <div>
        <asp:DropDownList ID="ddlCalender" runat="server" AutoPostBack="true" OnSelectedIndexChanged="ddlCalender_SelectedIndexChanged">
            <asp:ListItem Text="Please select" />
            <asp:ListItem Text="January 2013" />
            <asp:ListItem Text="Feb 2013" />
            <asp:ListItem Text="Mar 2013" />
            <asp:ListItem Text="April 2013" />
            <asp:ListItem Text="May 2013" />
            <asp:ListItem Text="Jun 2013" />
            <asp:ListItem Text="Jul 2013" />
            <asp:ListItem Text="Aug 2013" />
            <asp:ListItem Text="Sep 2013" />
            <asp:ListItem Text="Oct 2013" />
            <asp:ListItem Text="Nov 2013" />
            <asp:ListItem Text="Dec 2013" />
        </asp:DropDownList>
        <asp:TextBox ID="txtDate" runat="server" />
    </div>
    </form>

C#:

protected void ddlCalender_SelectedIndexChanged(object sender, EventArgs e)
        {
            string month = this.ddlCalender.SelectedItem.Text.Substring(0, 3);
            DateTime d = new DateTime();
            d = Convert.ToDateTime(this.ddlCalender.SelectedItem.Text);
            this.txtDate.Text = d.AddMonths(6).ToString(); ;
        }