As stated in the documentation, Cell.getCellStyle () will never return null.
https://poi.apache.org/apidocs/org/apache/poi/ss/usermodel/Cell.html#getCellStyle ()
If an explicit cell type is not set for a cell, it will return the default cell style, which is initially distributed among all the cells in the workbook. Changing this then, obviously, will affect all cells that do not have an explicitly assigned style.
You need to create a new CellStyle and then assign it to the appropriate cells.
In the POI Developer's Guide:
https://poi.apache.org/spreadsheet/quick-guide.html#CreateDateCells
Workbook wb = new HSSFWorkbook(); //Workbook wb = new XSSFWorkbook(); CreationHelper createHelper = wb.getCreationHelper(); Sheet sheet = wb.createSheet("new sheet"); // Create a row and put some cells in it. Rows are 0 based. Row row = sheet.createRow(0); // Create a cell and put a date value in it. The first cell is not styled // as a date. Cell cell = row.createCell(0); cell.setCellValue(new Date()); // we style the second cell as a date (and time). It is important to // create a new cell style from the workbook otherwise you can end up // modifying the built in style and effecting not only this cell but other cells. CellStyle cellStyle = wb.createCellStyle(); cellStyle.setDataFormat( createHelper.createDataFormat().getFormat("m/d/yy h:mm")); cell = row.createCell(1); cell.setCellValue(new Date()); cell.setCellStyle(cellStyle); //you can also set date as java.util.Calendar cell = row.createCell(2); cell.setCellValue(Calendar.getInstance()); cell.setCellStyle(cellStyle); // Write the output to a file FileOutputStream fileOut = new FileOutputStream("workbook.xls"); wb.write(fileOut); fileOut.close();