如何使用公历和循环计算两个日期之间的天数

public class DVD implements Serializable {

private static final long serialVersionUID = 1L;
protected GregorianCalendar rentedOn;
protected GregorianCalendar dueBack;
protected String title;
protected String nameOfRenter;

public double getCost(GregorianCalendar ActualDatRented) {
    double cost = 3;
    double count = 0;
    if (ActualDatRented.after(this.dueBack)) {
        while(!dueBack.equals(ActualDatRented)) {
            count++;
            dueBack.add(Calendar.DATE,1);
            cost = cost + count;
            return cost;
        }
    }else if(ActualDatRented.equals(dueBack))
        cost = 3;

    return cost;
  }
}
public class DVD implements Serializable {

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