setting the default value of a date type column of store from backend ?

[n_s_adhikari@rediffmail.com]

Hi
How can I set the default value of a date type column of store from backend ? I am trying set the defaultValue of DateTime column to a value which is determined by some logic.

<ext:Store ID="store1" runat="server">
        <Reader>
            <ext:JsonReader ReaderID="Id">
                <Fields>
                    <ext:RecordField Name="field12" />
                    <ext:RecordField Name="dt"  Type="Date" DeafultValue="............."/>
                    <ext:RecordField Name="Id" />
                </Fields>
            </ext:JsonReader>
        </Reader>
    </ext:Store>