time包有个函数Parse可以将时间字符串解析成Time对象，而Time对象有个Sub方法可以计算与某个时间的差，返回值是Duration对象，而Duration有一个Seconds方法，除以86400就是天数，再除以365就是多少年了

1. 计算日期相差多少个月

func MonthBetweenTwoDates(start, end string) int {
	startTime,_ := time.Parse("2006-01-02", start)
        endTime,_ := time.Parse("2006-01-02", start)
	months := 0
	month := startTime.Month()
	for startTime.Before(endTime) {
		startTime = startTime.Add(time.Hour * 24)
		nextMonth := startTime.Month()
		if nextMonth != month {
			months++
		}
		month = nextMonth
	}

	return months
}
fmt.Println(MonthBetweenTwoDates("2019-01-01", "2021-8-01"))

2.计算日期相差多少年

func YearBetweenTwoDates(start, end string) int {
	startTime,_ := time.Parse("2006-01-02", start)
        endTime,_ := time.Parse("2006-01-02", start)
	diff := endTime.Sub(startTime)
	return int(diff.Seconds() / 86400 / 365)
}

fmt.Println(YearBetweenTwoDates("2019-01-01", "2021-8-01"))