Al final lo resolví de la siguiente forma:
Código PHP:
$hora = date("H",PHPExcel_Shared_Date::ExcelToPHP($startTime));
$minutos = date("i",PHPExcel_Shared_Date::ExcelToPHP($startTime));
$mes = date("m",PHPExcel_Shared_Date::ExcelToPHP($startTime));
$dia = date("d",PHPExcel_Shared_Date::ExcelToPHP($startTime));
$year = date("y",PHPExcel_Shared_Date::ExcelToPHP($startTime));
$fechaFinalFormato = date("d/m/Y H:i",strtotime ( '-2 hour', mktime($hora,$minutos,0,$mes,$dia,$year)));