public double getDifferenceFromVocherInATime(String beginTime,String endTime){
		double totalDiff=0.0;
		
		ArrayList<Object> receipts=this.show();
		
		for (Iterator iterator = receipts.iterator(); iterator.hasNext();) {
			SalesReceiptPO object = (SalesReceiptPO) iterator.next();
			if(changeDateToInt(object.getTime())>=changeDateToInt(beginTime)&&changeDateToInt(object.getTime())<=changeDateToInt(endTime)){
				if(object.getSerialNumber().substring(0, 3).equals("XSD")&&(object.getVocher()>object.getFinalprice())){
					totalDiff+=(object.getVocher()-object.getFinalprice());
				}
			}
		}
		return totalDiff;
	}