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Datetime d1 = new Datetime();                    

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DateTime start = DateTime.Now;

/* ... Kaffee trinken gehen ... */

TimeSpan vergangen = DateTime.Now - start;                    

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        private void button1_Click(object sender, EventArgs e)
        {
            BackgroundWorker bw = new BackgroundWorker();
            bw.DoWork += new DoWorkEventHandler(kill);
            bw.RunWorkerAsync();
        }
        public void kill(object sender, DoWorkEventArgs e)
        {
            System.Windows.Forms.Timer t1 = new System.Windows.Forms.Timer();
            t1.Interval = 1000;
            t1.Tick += new EventHandler(zeig);
            t1.Enabled = true;
        }
        private void zeig(object sender, System.EventArgs e)
        {
             MessageBox.Show("test");
        }