To set to default Excel type Date (defaulted to OS level locale /-> i.e. xlsx will look different when opened by a German or British person/ and flagged with an asterisk if you choose it in Excel's cell format chooser) you should:
CellStyle cellStyle = xssfWorkbook.createCellStyle();
cellStyle.setDataFormat((short)14);
cell.setCellStyle(cellStyle);
I did it with xlsx and it worked fine.