Make sure you have set the following properties for formatting to work in Edit Mode in Bound field.
DataFormatString = ({0:d})
ApplyFormatInEditMode = True
In your code if you just modify like below, it should work fine
<asp:BoundField DataField="SNAPSHOT_DATA_DATE" HeaderText="Snapshot Date" ApplyFormatInEditMode="true" SortExpression="SNAPSHOT_DATA_DATE" DataFormatString="{0:d}" />