获取前一天、一周、一个月

public String getBefore(){ 
    SimpleDateFormat format= new SimpleDateFormat("yyyy-MM-dd");
    Date dateNow = new Date();
    Calendar calendar = Calendar.getInstance();
    calendar.setTime(dateNow);
    //calendar .add(Calendar.DAY_OF_YEAR, -1); //一天
    //calendar .add(Calendar.WEEK_OF_YEAR, -1); //一周
    calendar.add(Calendar.MONTH, -1); // 一个月
    Date dateFrom = calendar.getTime();
    String beforeTime=format.format(dateFrom);
    return beforeTime;
}