Populate DropDownList

I am having trouble w/ the dropdownlist. I am trying to set the values of the dropdownlist by the text. It is working ok(shows correct dates in view source). When the page loads it works but when I go to change it i get the error 
The string was not recognized as a valid DateTime. There is an unknown word starting at index 2. Also I debugged it and when I select another month dropdownlist.selected value in the where parameter turns into "1 month" instead of the date it was before. I can't figure this out. If I eliminate the code behind stuff and put actual dates as values in the dropdownlist everything works perfect. thanks, Jake
<asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True">
            <asp:ListItem Text="1 month"></asp:ListItem>
            <asp:ListItem Text="2 m ...