1

 public static String asd(int past, String time, Integer type) {
        //time  yyyy-MM-dd
        LocalDate now = LocalDate.now();
        Integer year = Integer.valueOf(time.substring(0, 4));
        Integer month = Integer.valueOf(time.substring(5, 7));
        if (type == 1) {
            //天
            Integer day = Integer.valueOf(time.substring(8, 10));
            LocalDate localDate 
 
= LocalDate.of(year, month, day);
            LocalDate localTimeDay = localDate.minusDays(past);
            return localTimeDay.toString();
        } else if (type == 2) {
            Integer day = 01;
            //月
            LocalDate localDate = LocalDate.of(year, month, day);
            LocalDate localTimeMonth 
 
= localDate.minusMonths(past);
            return localTimeMonth.toString().substring(0, 7);
        }
        return now.toString();
    }

但是這種也不算是很完美 按照下標擷取還是很不保險 ，測試的時候就發現 有人傳 2021-6-2 這種不是標準格式的。

所以有第二種方式。

2

/**
     * 獲取指定日期的前幾天或月份的日期
     *
     * @param past
     * @return
     */
    public
 
 static String getPastDate(int past, String time, Integer type) {
        //改為以- 擷取 防止出現 2021-6-2
        LocalDate now = LocalDate.now();
        int index = time.indexOf("-");
        int indexOf = time.indexOf("-", index + 1);
        Integer year = Integer.valueOf(time.substring(0, time.indexOf("-")));
        Integer month = Integer.valueOf(time.substring(index + 1, indexOf));
        if (type == 1) {
            //天
            Integer day = Integer.valueOf(time.substring(indexOf + 1));
            LocalDate localDate = LocalDate.of(year, month, day);
            LocalDate localTimeDay = localDate.minusDays(past);
            return localTimeDay.toString();
        } else if (type == 2) {
            Integer day = 01;
            //月
            LocalDate localDate = LocalDate.of(year, month, day);
            LocalDate localTimeMonth = localDate.minusMonths(past);
            return localTimeMonth.toString().substring(0, 7);
        }
        return now.toString();
    }