How to set an Expiry Date for web application using ASP.NET C#

-----------Web.config -------
 <appSettings>
    <add key="copyright" value="C# Soft Code © 2019"/>
    <add key="RegDate" value="01/01/2018"/>
    <add key="ExpDateMess" value="01/01/2020"/>
    <add key="ExpDate" value="01/02/2020"/>
 
  </appSettings>

-----------------aspx.cs code file-------------

 private void showMessage()
    {

        DateTime expDate = Convert.ToDateTime(ConfigurationManager.AppSettings["ExpDateMess"].ToString());
        DateTime currentDate = Convert.ToDateTime(DateTime.Now.ToShortDateString());
        TimeSpan t = expDate - currentDate;
        double nrOfDays = t.TotalDays;

        if (nrOfDays > 0)
        {

        }
        else
        {
            ClientScript.RegisterStartupScript(typeof(Page), "alertMessage", "<script type='text/javascript'>alert('Your software key is expired. Please contact your software provider for renewal.');</script>");
        }

    }
    protected void lbtnLogin_Click(object sender, EventArgs e)
    {

        if (string.IsNullOrEmpty(txtUserName.Text))
        {

            ClientScript.RegisterStartupScript(this.GetType(), "Message", "<script>alert('Please enter user Name..!')</script>");
            txtUserName.Focus();
            return;
        }
        if (string.IsNullOrEmpty(txtPassword.Text))
        {

            ClientScript.RegisterStartupScript(this.GetType(), "Message", "<script>alert('Please enter Password ..!')</script>");
            txtPassword.Focus();
            return;
        }

        DateTime expDate = Convert.ToDateTime(ConfigurationManager.AppSettings["ExpDate"].ToString());
        DateTime currentDate = Convert.ToDateTime(DateTime.Now.ToShortDateString());
        TimeSpan t = expDate - currentDate;
        double nrOfDays = t.TotalDays;



        if (nrOfDays > 0)
        {

            ClientScript.RegisterStartupScript(this.GetType(), "Message", "<script>alert('Welcome to C# Soft Code')</script>");



        }
        else
        {
            ClientScript.RegisterStartupScript(this.GetType(), "Message", "<script>alert('Your software key expired. Please contact your software provider for renewal.!')</script>");

        }
    }