The trick mentioned in the http://officeopenxml.com/SScontentOverview.php doesn't seem to work for me as well.
If I create in excel a cell with date formatting. It defines the date format using style in the file xl\styles.xml

<cellXfs count="2"> <xf numFmtId="0" fontId="0" fillId="0" borderId="0" xfId="0"/> <xf numFmtId="14" fontId="0" fillId="0" borderId="0" xfId="0" applyNumberFormat="1"/> </cellXfs>

If I use the same style for the cell (<c s='1'...) it properly formats the date.
https://playground.jsreport.net/w/anon/Uq8QQFxW