Unless you're using PHPExcel's "Advanced Value Binder", then $sheet->setCellValueByColumnAndRow($column, $row, '2010-07-16'); will be storing the value as a string, not as a date, so the date format you're setting in the next line is meaningless when applied to a string until you read the resultant file in Excel and force a refresh... Excel itself then fixes your error.
To ensure that the value is correctly stored in the first place, you need to store it as a date/timestamp/number rather than a string, then set the format mask to ensure that it is treated as a date/timestamp rather than a numeric value.
Either convert your string to a PHP date using strtotime(), then use PHPExcel's built in date conversion methods:
$PHPDateValue = strtotime('2010-07-16');
$ExcelDateValue = PHPExcel_Shared_Date::PHPToExcel($PHPDateValue);
$sheet->setCellValueByColumnAndRow($column, $row, $ExcelDateValue);
$sheet->getStyleByColumnAndRow($column, $row)->getNumberFormat()->setFormatCode(PHPExcel_Style_NumberFormat::FORMAT_DATE_DMYSLASH);
or use the built-in method to convert a date-formatted string to an Excel datetime value directly
$dateString = '2010-07-16';
$ExcelDateValue = PHPExcel_Shared_Date::stringToExcel($dateString);
$sheet->setCellValueByColumnAndRow($column, $row, $ExcelDateValue);
$sheet->getStyleByColumnAndRow($column, $row)->getNumberFormat()->setFormatCode(PHPExcel_Style_NumberFormat::FORMAT_DATE_DMYSLASH);
or use the Calculation Engines function library's DATEVALUE() function:
$dateString = '2010-07-16';
$ExcelDateValue = PHPExcel_Calculation_Functions::DATEVALUE($dateString);
$sheet->setCellValueByColumnAndRow($column, $row, $ExcelDateValue);
$sheet->getStyleByColumnAndRow($column, $row)->getNumberFormat()->setFormatCode(PHPExcel_Style_NumberFormat::FORMAT_DATE_DMYSLASH);
or, option 4, is to use PHPExcel's "Advanced Value Binder"
To enable this feature, execute the following static call
PHPExcel_Cell::setValueBinder( new PHPExcel_Cell_AdvancedValueBinder() );
before you instantiate your workbook object, or load it from file
Then PHPExcel will identify that your value is a date, and handle the conversion to an Excel date/timestamp and format it automatically
$dateString = '2010-07-16';
$sheet->setCellValueByColumnAndRow($column, $row, $dateString);