BusinessHours.diff

datafim:2016-09-20 00:00:00

public static integer calcula(String id,DateTime datainicial,DateTime datafim){
	  	BusinessHours bh = [SELECT id  From BusinessHours Where id=:id];
		Long businessDiff = BusinessHours.diff(bh.Id, datainicial, datafim);
		System.debug('businessDiff:'+businessDiff);
		String strDataInicial = datainicial.format('EEE');
        String strDataFim = datafim.format('EEE');
        System.debug('strDataInicial:'+strDataInicial);
        System.debug('strDataFim:'+strDataFim);  
		Long seconds = businessDiff / 1000;
		Long minutes = seconds / 60;
		Long hours = minutes / 60;
		Long dia = hours / 24;
		System.debug('diaCalculado'+dia);
		return dia.intValue();
  	}

The code is putting the start date to Sunday and it is a Monday, so I'm always with return 0.