1、DateTime

         DateTime now = System.DateTime.Now;
         now.ToString();                                   //显示: 2006/08/30 17:31:02
         now.ToString("yyyy-mm-dd hh:MM:ss");              //显示: 2006-08-30 05:39:11
         now.ToString("yyyy-mm-dd HH:mm:ss");              //显示: 2006-08-30 17:40:50
         System.DateTime.MaxValue.ToString();              //显示: 9999/12/31 23:59:59
         System.DateTime.MinValue.ToString();              //显示: 0001/01/01 0:00:00
         now.ToLongDateString();                           //显示: 2006年8月30日
         now.ToLongTimeString();                           //显示: 17:34:23
         now.ToShortTimeString();                          //显示: 17:34
         now.ToString() + " " + now.Millisecond.ToString();//显示: 2006/08/30 17:35:19 484

2、程序运行时间:(单位 : 毫秒)

        System.Diagnostics ;  //名称空间
        int x = 0;
        int nu = 0;
        Stopwatch sw = new Stopwatch();
        sw.Start();
        //程序开始
        for (int i = 0; i < 1000000; i++)
        {
             x += i;
        }
        //程序结束
        sw.Stop();
        this.label1.Text += ",sum=" + x.ToString();
        MessageBox.Show(sw.ElapsedMilliseconds.ToString());

3、计算一个页面的执行时间

        protected void Application_BeginRequest(Object sender, EventArgs e)
        {
            Application["StartTime"] = System.DateTime.Now;
        }
        protected void Application_EndRequest(Object sender, EventArgs e)
        {
            System.DateTime startTime = (System.DateTime)Application["StartTime"];
            System.DateTime endTime = System.DateTime.Now;
            System.TimeSpan ts = endTime - startTime;
            Response.Write("页面执行所用时间:" + ts.Milliseconds + " 毫秒");
        }

4、计算程序实际运行时间

获得时间差（单位毫秒）

//StartTime在程序初始化时获得，存于内存中
            while (true)
            {
                //lbl_NowTime.Text = DateTime.Now.Hour + ":" + DateTime.Now.Minute + ":" + DateTime.Now.Second;
                try
                {
                    lbl_NowTime.Text = DateTime.Now.ToLongTimeString();
                    lbl_StartTime.Text = formatMsecToTimeStr((DateTime.Now - StartTime).TotalMilliseconds);
                }
                catch (Exception)
                {
                    ;
                }                               
                Thread.Sleep(50);
            } 

根据毫秒数计算时间函数

        /// <summary>
        /// 根据输入的毫秒，返回一个xx:xx:xx格式的字符串
        /// </summary>
        /// <param name="Msec">毫秒数</param>
        /// <returns>返回xx:xx:xx格式的字符串</returns>
        public string formatMsecToTimeStr(double Msec)
        {
            int hour = 0;
            int minute = 0;
            int second = 0;
            second = (int)(Msec / 1000);//将毫秒转换成分钟

            if (second > 60)
            {
                minute = second / 60;//分钟取模
                second = second % 60;//秒钟取余
            }
            if (minute > 60)
            {
                hour = minute / 60;//小时取模
                minute = minute % 60;//分钟取余
            }

            //如果数字小于两位数，则字符串前加0
            string strHour="", strMinute="", strSecond="";
            if (hour<10)
            {
                strHour += ("0"+ hour.ToString());
            }
            else
            {
                strHour += hour.ToString();
            }

            if (minute < 10)
            {
                strMinute += ("0" + minute.ToString());
            }
            else
            {
                strMinute += minute.ToString();
            }

            if (second < 10)
            {
                strSecond += ("0" + second.ToString());
            }
            else
            {
                strSecond += second.ToString();
            }


            return $"{strHour}:{strMinute}:{strSecond}";
        }