Java Data Type - 如何将一个月添加到日期

import java.util.GregorianCalendar;
import java.util.Calendar;

public class Main {
    public static void main(String[] args) {

        Calendar today = new GregorianCalendar();
        Calendar purchased = new GregorianCalendar();

        purchased.add(Calendar.DAY_OF_MONTH, 1);

        Calendar lastActDate = new GregorianCalendar();
        lastActDate.setTimeInMillis(today.getTimeInMillis() - purchased.getTimeInMillis());
    }
}