Error on 'Timer' is an ambiguous reference between 'System.Windows.Forms.Timer' and 'System.Threading.Timer'

I have encountered this .NET compile time error, as shown below:

'Timer' is an ambiguous reference between 
'System.Windows.Forms.Timer' and 'System.Threading.Timer' 

It occurred after I added this code block for a clock:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Net.Sockets;
using System.Threading;
using System.Security.Cryptography;


namespace SocketClient
{

    public partial class SocketClient : Form
    {
        System.Net.Sockets.TcpClient clientSocket = new System.Net.Sockets.TcpClient();
        NetworkStream serverStream = default(NetworkStream);
        string readData = null;


        public SocketClient()
        {
            InitializeComponent();
            Timer timer = new Timer();
            timer.Tick += new EventHandler(TimerOnTick);
            timer.Interval = 1000;
            timer.Start();
        }

         

        private void TimerOnTick(object sender, EventArgs ea)
        {
            Invalidate();
        }

        protected override void OnPaint(PaintEventArgs pea)
        {
            StringFormat strfmt = new StringFormat();
            strfmt.Alignment = StringAlignment.Far;
            strfmt.LineAlignment = StringAlignment.Far;

            pea.Graphics.DrawString(DateTime.Now.ToString("F"),
            Font, new SolidBrush(ForeColor),
            ClientRectangle, strfmt);

        }

        private void getMessage()
        {
            while (true)
            {
                serverStream = clientSocket.GetStream();
                int buffSize = 0;
                byte[] inStream = new byte[10025];
                buffSize = clientSocket.ReceiveBufferSize;
                serverStream.Read(inStream, 0, buffSize);
                string returndata = System.Text.Encoding.ASCII.GetString(inStream);