Date is not a date

I am wrestling with a comparevalidator. the value of the textbox expireson is not supposed to be later than 30 days after the value of createdon.

In the markup code:
            Created On: <asp:Label ID="createdon" runat="server"  /><br />
            Expires: <asp:TextBox id="expiresOn" runat="server" />
            <asp:RequiredFieldValidator ID="reqExpiresOn" runat="server" ControlToValidate="expiresOn" text="(Required)" />
            <asp:compareValidator  id="cmpvExp1"
                  ControlToValidate="expireson"
    &nb ...