Sorting of date column in GridView ASP.Net

I want to Sort this Invoice date column to go from the latest date "July 2015" to the oldest. I can't figure out how sort the Date Column. Any help would be really appreciated. Here is my code <asp:BoundField DataField="Month" DataFormatString="{0:Y}" HeaderText="Invoice Date" HtmlEncode="False"/>

via Chebli Mohamed