参照渡し
class func addMonthWithYearMonth(inout ym:(year:Int,month:Int),addMonth:Int){
    var y:Int = ym.year
    var m:Int = ym.month + addMonth
    while 12 < m {
        m -= 12
        y++
    }
    while m < 1 {
        m += 12
        y--
    }
    ym.year=y
    ym.month=m
}