Convert java DateFormat patterns into Excel custom number formats. For example, to format a date in excel using the "dd MMMM, yyyy" pattern and Japanese locale, use the following code:

     // returns "[$-0411]dd MMMM, yyyy;@" where the [$-0411] prefix tells Excel to use the Japanese locale
     String excelFormatPattern = DateFormatConverter.convert(Locale.JAPANESE, "dd MMMM, yyyy");

     CellStyle cellStyle = workbook.createCellStyle();

     DataFormat poiFormat = workbook.createDataFormat();
     cellStyle.setDataFormat(poiFormat.getFormat(excelFormatPattern));
     cell.setCellValue(new Date());
     cell.setCellStyle(cellStyle);  // formats date as '2012年3月17日'
 

TODO Generalise this for all Excel format strings