一, 声明开始时间和结束时间

       let create_time = "2016-08-12 8:34:21"
        let end_time = "2016-08-14 09:32:08"

二, 声明方法将秒数转为 时:分:秒

func getHHMMSSFormSS(seconds:Int) -> String {
        let str_hour = NSString(format: "%02ld", seconds/3600)
        let str_minute = NSString(format: "%02ld", (seconds%3600)/60)
        let str_second = NSString(format: "%02ld", seconds%60)
        let format_time = NSString(format: "%@:%@:%@",str_hour,str_minute,str_second)
        return format_time as String
    }

三, 计算时间差

       let dateFormatter = NSDateFormatter()
        dateFormatter.dateFormat = "yyyy-MM-dd HH:mm:ss"
        let timeNumber = Int(dateFormatter.dateFromString(end_time)!.timeIntervalSince1970-dateFormatter.dateFromString(create_time)!.timeIntervalSince1970)
        print(self.getHHMMSSFormSS(timeNumber))

四, 最终结果

48:57:47