DateTime dt = DateTime.Now;//2020/5/10 22:42:43
            //想要什么格式 format输入想要的格式
            string str= string.Format("{0:yyyy-MM-dd}", dt);//2020-5-10
            //定义的时候格式化
            string dt2= DateTime.Now.ToString("yyyy-MM-dd HH:mm");//2020-5-10 22:42
            DateTime currentTime = System.DateTime.Now;
            int year = currentTime.Year;//年
            int Hour = currentTime.Hour;//小时
            DateTime.Now.DayOfWeek.ToString();//星期几
            string str1 = "2020-05-07";
            //string类型转换成datetime
            DateTime dt1 = Convert.ToDateTime(str1);