c# winform timer计时器停止并重置计时器

       最近在做一个倒计时的计时器，是用timer控件进行控制的。倒计时的部分是没有问题的，但是现在需要实现点击btn后，停止当前的计时，并且重置计时器为计时的初始状态这项功能。详见代码。
      

      

        private void btn_end_Click(object sender, EventArgs e)

        {

            timer1.Stop();

            timer1.Enabled = false;

            timer1.Dispose();

        }



        long sec = 1800;

        private void timer1_Tick(object sender, EventArgs e)

        {

            

            sec -= 1;

            this.lb_time.Text= new DateTime(sec * 1000000).ToLongTimeString();

            if (sec == 300 )

            {

               

                this.lb_zff.Text  = "最后30秒!";

                

               

                this.axWindowsMediaPlayer1.URL = "D:\\ding.wav";

            }

           

            if (sec == 0)

            {

                timer1.Stop();

                this.lb_zff.Text = "时间到！";

                this.axWindowsMediaPlayer1.URL= "D:\\ling.wav";



            }



              private void btn_start_Click(object sender, EventArgs e)

        {

            if (timer1.Enabled)

            {

                timer1.Stop();

           

            }

            else

            {