Display date in TextBox from Day, Month, Year DropDownList in ASP.Net

Create a single OnSelected Index Changed Event

HTML

<form id="form1" runat="server">
<div>
    <asp:DropDownList ID="ddlDays" runat="server" AutoPostBack="true" OnSelectedIndexChanged="OnSelectedIndexChangedGetDate">
        <asp:ListItem Text="01" Value="01" />
        <asp:ListItem Text="02" Value="02" />
        <asp:ListItem Text="03" Value="03" />
        <asp:ListItem Text="04" Value="04" />
    </asp:DropDownList>
    <br />
    <asp:DropDownList ID="ddlMonths" runat="server" AutoPostBack="true" OnSelectedIndexChanged="OnSelectedIndexChangedGetDate">
        <asp:ListItem Text="Jan" Value="01" />
        <asp:ListItem Text="Feb" Value="02" />
        <asp:ListItem Text="Mar" Value="03" />
        <asp:ListItem Text="Apr" Value="04" />
    </asp:DropDownList>
    <asp:DropDownList ID="ddlYears" runat="server" AutoPostBack="true" OnSelectedIndexChanged="OnSelectedIndexChangedGetDate">
        <asp:ListItem Text="2001" Value="2001" />
        <asp:ListItem Text="2002" Value="2002" />
        <asp:ListItem Text="2003" Value="2003" />
        <asp:ListItem Text="2004" Value="2004" />
    </asp:DropDownList>
    <hr />
    <asp:TextBox ID="txtDate" runat="server" />
</div>
</form>

 C#

protected void OnSelectedIndexChangedGetDate(object sender, EventArgs e)
{
    this.txtDate.Text = string.Format("{0}/{1}/{2}", this.ddlDays.SelectedItem.Value, this.ddlMonths.SelectedItem.Value, this.ddlYears.SelectedItem.Value);
}