If you want the whole worksheet, then read using Excel2007, select required worksheet, and write using CSV... as long as you don't set readDataOnly(true), then that's the simplest 5 or 6 lines to do a conversion between formats.
$objReader = PHPExcel_IOFactory::createReader('Excel2007');
$objPHPExcel = $objReader->load($inputFileName);
$objWriter = PHPExcel_IOFactory::createWriter($objPHPExcel, 'CSV');
And this will handle all date conversions for you, using the formats identified in the Excel worksheet.
To Excel, a date is just a number, hence the return from getDataType().
If you want to test a cell to identify if it's a date, perhaps to change the date formatting:
$isDate = PHPExcel_Shared_Date::isDateTime($objWorksheet->getCellByColumnAndRow($col, $row));