Conversion failed when converting date and/or time from character string. System.Data.SqlClient.SqlException:
<p>Every time I hit logout button I get this error 
System.Data.SqlClient.SqlException: Conversion failed when converting date and/or time from character string.</p>

<blockquote>
  <p>This my Coding Part. Help me.</p>
</blockquote>

<pre><code>  SqlConnection conlog = new SqlConnection(ConfigurationManager.ConnectionStrings["loginConnectionString"].ConnectionString);
        conlog.Open();
    Session["New"] = null;

    string logintime = "select LoginTime from Logindata where UserName ='" + Label1.Text + "'";
    SqlCommand logintimequery = new SqlCommand(logintime, conlog);
    int logintimestr = logintimequery.ExecuteNonQuery();


        string idQuery = "Select ID from [Table] where Username='"+Label1.Text+"'";
    SqlCommand idd = new SqlCommand(idQuery, conlog);
    string strQuery = "update logindata set LogoutTime='" + DateTime.Now + "' where LoginTime='"+logintimestr+ "' and UserName='" + Label1.Text + "'  ";
    SqlCommand cmd = new SqlCommand(strQuery, conlog);

    cmd.Parameters.AddWithValue("@uname", Label1.Text);
    cmd.Parameters.AddWithValue("@logouttime", DateTime.Now.ToString());
    cmd.ExecuteNonQuery();
    Response.Redirect("Loginform.aspx");
    conlog.Close();
</code></pre>