swift 获取上一个月份(年份)

分类中写

    static func getLastMonth(num : Int) -> String {
        let curDate = Date()
        let formater = DateFormatter()
        formater.dateFormat = "MM"
        
        let calendar = Calendar(identifier: .gregorian)
        var lastMonthComps = DateComponents()
        // year = 1表示1年后的时间 year = -1为1年前的日期，month day 类推
        lastMonthComps.month = num
        let newDate = calendar.date(byAdding: lastMonthComps, to: curDate)
        let dateStr = formater.string(from: newDate!)
        
        return dateStr
    }