How can i add Time stamp for serial port for sending data and receiving

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I am able to Transfer data and files over serial port in c# , but the problem is i have to send the file in 500 ms can anyone suggest me how to send the file within time period and how to add time stamp

here my code for sending files

   private void butSendFile_Click(object sender, EventArgs e)
        {
            if (serialPort1.IsOpen == false)
            {
                this.serialPort1.Open();
            }
            serialPort1.Write(System.IO.File.ReadAllText(this.txtfile.Text));
        }
 
        private void butSetFile_Click(object sender, EventArgs e)
        {
 
            OpenFileDialog ofd = new OpenFileDialog();
            if (ofd.ShowDialog() == DialogResult.OK)
            {
                txtfile.Text = ofd.FileName;
            }
        }