using System;
using System.Collections.Generic;
using
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace ClockAlarm
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            string strHour = DateTime.Now.TimeOfDay.Hours.ToString();
            string strMouit = DateTime.Now.TimeOfDay.Minutes.ToString();
            string strSecon = DateTime.Now.TimeOfDay.Seconds.ToString();
            if (Convert.ToInt32(strHour) < 10)
            {
                strHour = "0" + strHour;
            }
            if (Convert.ToInt32(strMouit) <10)
            {
                strMouit = "0" + strMouit;
            }
            if (Convert.ToInt32(strSecon) < 10)
            {
                strSecon = "0" + strSecon;
            }
            textBox2.Text = strHour + ";" + strMouit + ":" + strSecon;
            int Hour = Convert.ToInt32(strHour);
            int Mouit = Convert.ToInt32(strMouit);
            int Secon = Convert.ToInt32(strSecon );
            numericUpDown3.Value = Convert.ToInt32(strHour );
            numericUpDown2.Value = Convert.ToInt32(strSecon );
            numericUpDown1.Value = Convert.ToInt32(strSecon );
        }
         
        private void timer1_Tick(object sender, EventArgs e)
        {
            string strHour = DateTime.Now.TimeOfDay.Hours.ToString();
            string strMouit = DateTime.Now.TimeOfDay.Minutes.ToString();
            string strSecon = DateTime.Now.TimeOfDay.Seconds.ToString();
            if (Convert.ToInt32(strHour) < 10)
            {
                strHour = "0" + strHour;
            }
            if (Convert.ToInt32(strMouit) < 10)
            {
                strMouit = "0" + strMouit;
            }
            if (Convert.ToInt32(strSecon) < 10)
            {
                strSecon = "0" + strSecon;
            }
            textBox1.Text = strHour + ":" + strMouit + ":" + strSecon;
        }

        private void button1_Click(object sender, EventArgs e)
        {
            DateTime get_time1 = Convert.ToDateTime(DateTime.Now.ToString());
            DateTime sta_ontime1 = Convert.ToDateTime(Convert.ToDateTime(textBox2.Text.Trim().ToString()));
            long dat = DateAndTime.DateDiff("s",get_time1 ,sta_ontime1 ,FirstDayOfWeek.Sunday,FirstWeekOfYear.FirstFourDays);//此处我有错误
            if (dat > 0)
            {
                timer2.Enabled = true;
                label4.Text = "闹钟已启动";
                label1.Text = "剩余" + dat.ToString() + "秒";
            }
            else
            {
                MessageBox.Show("闹钟已启动，请取消后，再启动");
            }
            else//此处？？？
            {
                long hour =24*3600+dat;
                timer2 .Enabled =true;
                label4 .Text ="闹钟已启动";
                label1 .Text ="剩余"+hour .ToString ()+"秒";
            }
        }

        private void timer2_Tick(object sender, EventArgs e)
        {
            DateTime get_time1 = Convert.ToDateTime(DateTime .Now .ToString ());
            DateTime sta_ontime1 = Convert.ToDateTime(Convert .ToDateTime (textBox2 .Text .Trim ().ToString ()));
            long dat = DateAndTime.DateDiff("s", get_time1, sta_ontime1, FirstDayOfWeek.Sunday, FirstWeekOfYear.FirstFourDays);//此处我有错误
            if (dat == 0)
            {
                label4.Text = "时间已到";
                timer2.Enabled = false;
            }
            else
            {
                label1.Text = "剩余" + dat.ToString() + "秒";
            }
        }

    }
}
代码如上，DateAndTime.DateDiff百度过，语法没错，但不知为什么就出错了，还望大神指教，谢谢了

少了引用集