如果不能使用java8，建议使用Calendar类来做，代码如下，add方法第二个参数传递负值即可。

        Calendar now = Calendar.getInstance();
        now.add(Calendar.DAY_OF_MONTH, -30);
        String endDate = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").format(now.getTime());
        System.out.println(endDate);

java8版本可以使用下面的方法：

        LocalDateTime now = LocalDateTime.now();
        now = now.minus(30, ChronoUnit.DAYS);
        System.out.println(now.toString());

如下，最后格式根据需要转化下即可：

var startDate = new Date( new Date() - 24*60*60*1000 * 30 );
console.log('startDate: ' + startDate);
// startDate: Tue Jan 19 2016 15:42:19 GMT+0800 (CST)

startDate = nowDate - 30 * 24 * 60 * 60 * 1000

现在时间减去三十天的毫秒数即可。

Date endDate = new Date();
Date startDate = new Date(endDate.getTime() - 30 * 24 * 60 * 60 * 1000);

public class Test {
    public static void main(String[] args) {
        Date endDate = new Date();
        Date startDate = getDateBefore(new Date(),30);
    }

    private static Date getDateBefore(Date d, int day){
        Calendar now =Calendar.getInstance();
        now.setTime(d);
        now.set(Calendar.DATE,now.get(Calendar.DATE)-day);
        return now.getTime();
    }
}

import org.apache.commons.lang3.time.DateUtils;

Date now = new Date();
Date startDate = DateUtils.addDays(now, -30);