I generate an XML file through C#, and the relevent part looks like this
2009-01-18T00:00:00.000
In the <Styles>
section add
<Style ss:ID="s22">
<NumberFormat ss:Format="yyyy\-mm\-dd"/>
</Style>
then mark your cell with the style:
<Cell ss:StyleID="s22"><Data ss:Type="DateTime">2009-01-21T00:00:00.000</Data></Cell>
Alternatively, open your XML file in Excel, make the change you want, re-save it as XML, then look to see what changed.