Get Items from ListBox and Insert Into DropDown List

Hi,

does anyone know how do I get the selected values from Listbox and insert them into DropDown List, then the values are being displayed from latest to oldest date?

This is what I have:

ListBox:

<asp:ListBox ID="DateListBox" runat="server" AppendDataBoundItems="True" SelectionMode="Multiple">
<asp:ListItem Text="01 Jan 2015" Value ="1" />
<asp:ListItem Text="01 Feb 2015" Value ="2" />
<asp:ListItem Text="01 Mar 2015" Value ="3" />
<asp:ListItem Text="01 Apr 2015" Value ="4" />
</asp:ListBox>

DropDown List:

<asp:Dropdownlist ID="DateDropDownList" runat="server" Width="136px" AutoPostBack ="True" AppendDataBoundItems="true">
</asp:Dropdownlist>

Once I click on the Retrieve button, based on selected values from the ListBox, only the Selected values are being displayed onto the DropDown List from latest to the oldest date.

Any help is greatly appreciated. Thanks!