计算日期间隔自然月份和天数

给定两个时间，startTime和endTime。计算这两个时间的间隔月份数，不是整月计算天数，月份数按照自然月来计算。举几个例子：

startTime:2022-02-16 00:00:00

endTime:2022-06-15 23:59:59

这两个时间间隔4个月。

startTime:2022-02-16 00:00:00

endTime:2022-08-27 23:59:59

这两个时间间隔6个月12天。

startTime:2022-02-16 00:00:00

endTime:2022-02-28 23:59:59

这两个时间间隔13天。

这里要注意，23:59:59相当于第二天的凌晨了。

我将月份数和天数转换为了整数部分和小数部分。比如：12个月20天，转换为：12.20。如果不需要这种格式，可以自己处理。

下面看代码

public double monthInterval(Date startTime,Date endTime){
    Calendar startCalendar = Calendar.getInstance();
    startCalendar.setTime(startTime);

    Calendar endCalendar = Calendar.getInstance();
    endCalendar.setTime(endTime);

    // 23:59:59 算第二天凌晨
    startCalendar.add(Calendar.SECOND, 1);
    endCalendar.add(Calendar.SECOND, 1);

    double finalMathInterval;

    // 数据校验
    if (startTime.after(endTime)) {
        // 错误日期
        return -999.9;
    }

    int startYear = startCalendar.get(Calendar.YEAR);
    int endYear = endCalendar.get(Calendar.YEAR);
    int startMonth = startCalendar.get(Calendar.MONTH) + 1;
    int endMonth = endCalendar.get(Calendar.MONTH) + 1;
    int startDay = startCalendar.get(Calendar.DAY_OF_MONTH);
    int endDay = endCalendar.get(Calendar.DAY_OF_MONTH);

    int diffYear = endYear - startYear;

    if (endMonth < startMonth && diffYear != 0) {
        diffYear = diffYear - 1;
    }

    // 天数相等
    if (startDay == endDay) {
        if (endMonth < startMonth) {
            finalMathInterval = 12 - startMonth + endMonth;
        } else {
            finalMathInterval = diffYear * 12 + endMonth - startMonth;
        }
    } else {
        int diffMonth = endMonth - startMonth;
        if (endMonth < startMonth) {
            diffMonth = 12 - startMonth + endMonth;
        }
        if (endDay < startDay) {
            diffMonth = diffMonth - 1;
        }

        int monthInterval = diffYear * 12 + diffMonth;

        // 计算相差天数
        // 预期月份后的日期
        startCalendar.add(Calendar.MONTH, monthInterval);
        int actualMaxDay = startCalendar
            .getActualMaximum(Calendar.DAY_OF_MONTH);
        int expectMonth = startCalendar.get(Calendar.MONTH) + 1;

        int diffDay;
        double diffDayDouble;
        if (expectMonth == endMonth) {
            diffDay = endDay - startDay;
        } else {
            diffDay = actualMaxDay - startDay + endDay;
        }
        if (diffDay <= 10) {
            diffDayDouble = new BigDecimal(diffDay).divide(
                new BigDecimal("10"), 1, RoundingMode.HALF_UP)
                .doubleValue();
        } else {
            diffDayDouble = new BigDecimal(diffDay).divide(
                new BigDecimal("100"), 2, RoundingMode.HALF_UP)
                .doubleValue();
        }

        finalMathInterval = monthInterval + diffDayDouble;
    }
    return finalMathInterval;
}