POI API

Add date into cell with formating

// create a new cell style from the workbook.
CellStyle cellStyle = workbook.createCellStyle();
CreationHelper createHelper = workbook.getCreationHelper();
// Set the date format of date
cellStyle.setDataFormat(createHelper.createDataFormat().getFormat(
        "dd/MM/yyyy hh:mm:ss"));
cell = row.createCell(1);
cell.setCellValue(new Date());
cell.setCellStyle(cellStyle);     

Set date by using Calendar

// You also can set date as java.util.Calendar object.
cell = row.createCell(2);
cell.setCellValue(Calendar.getInstance());


Check cell type (using HSSFDateUtil)

String JAVA_TOSTRING = "EEE MMM dd HH:mm:ss zzz yyyy";
Object value = "";
switch(cell.getCellType())
{                          
case HSSFCell.CELL_TYPE_NUMERIC:
value = cell.getNumericCellValue();
if(HSSFDateUtil.isCellDateFormatted(cell))
{
    if(HSSFDateUtil.isValidExcelDate(value))
    {
        Date date = HSSFDateUtil.getJavaDate(value);
        SimpleDateFormat dateFormat = new SimpleDateFormat(JAVA_TOSTRING);  
        value = dateFormat.format(date);                              
    }
    else
    {
        throw new Exception("Invalid Date value found at row number " +
                row.getRowNum()+" and column number "+cell.getCellNum());  
    }
}
else
{
    value = value + "";
}
break;

case HSSFCell.CELL_TYPE_STRING:
    value = cell.getStringCellValue();
break;

case HSSFCell.CELL_TYPE_BLANK:
    value = null;  
break;

Evaluate formula of Workbook

// evaluate all formula cells in workbook
 HSSFFormulaEvaluator.evaluateAllFormulaCells(workbook);