Java Data Type - 如何获得6个月前的日期

import java.util.Calendar;

public class Main
{
    public static void main(String[] args)
    {
        Calendar cal1 = Calendar.getInstance();
        cal1.set(2015, 2, 27);
        Calendar cal2 = Calendar.getInstance();
        cal2.set(2015, 8, 1);
        boolean valid = isSixMonthsAgo(cal1, cal2);
        System.out.println(valid);
    }

    public static boolean isSixMonthsAgo(Calendar c1, Calendar c2) 
    {
        int year1 = c1.get(Calendar.YEAR);
        int month1 = c1.get(Calendar.MONTH);

        int year2 = c2.get(Calendar.YEAR);
        int month2 = c2.get(Calendar.MONTH);

        if ((year1 * 12 + month1) - (year2 * 12 + month2) > 6)
            return true;
        return false;
    }
}