Swift版获取下一月或上一月，若跨年则变化年份

    //每调用一次月份向下跳一月，跨年则为下一年1月
    func getNextYearAndMonth () -> (year:Int,month:Int){
        var next_year:Int = currentYear
        var next_month:Int = currentMonth + 1
        if next_month > 12 {
            next_month=1
            next_year++
        }
        //Swift可以同时返回多个值，返回值类型为元组
        return (next_year,next_month)
    }

    //每调用一次月份向上跳一月，跨年则为上一年12月
    func getPrevYearAndMonth () -> (year:Int,month:Int){
        var prev_year:Int = currentYear
        var prev_month:Int = currentMonth - 1
        if prev_month == 0 {
            prev_month = 12
            prev_year--
        }
        return (prev_year,prev_month)
    }