How to use timers to remind my events on a time specified by user?

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace TimerExample
{
    public partial class Form1 : Form
    {
        private List<tuple><datetime,>> appointmentList;
        private Timer timer;

        public Form1()
        {
            InitializeComponent();
            lblMessage.Text = "";
        }

        private void btnStart_Click(object sender, EventArgs e)
        {
            // Create a list of test calendar items
            CreateCalendarItems();

            // Create the timer and start it
            CreateTimer();

            lblMessage.Text = "Timer Started";
            SetTime();
        }

        private void CreateCalendarItems()
        {
            appointmentList = new List<tuple><datetime,>>();

            // Add 3 example appointments to the list
            appointmentList.Add(new Tuple<datetime,string>(DateTime.Now.AddMinutes(1), "Budget meeting"));
            appointmentList.Add(new Tuple<datetime,string>(DateTime.Now.AddMinutes(2), "Marketing meeting"));
            appointmentList.Add(new Tuple<datetime,string>(DateTime.Now.AddMinutes(3), "Project Review"));
        }

        private void SetTime()
        {
            txtTime.Text = DateTime.Now.ToLocalTime().ToShortTimeString();
        }

        private void CreateTimer()
        {
            timer = new Timer();
            timer.Interval = 10000;     // 10k milliseconds equal 10 seconds
            timer.Tick += timer_Tick;
            timer.Start();
        }

        private void timer_Tick(object sender, EventArgs e)
        {
            SetTime();

            var appointments = appointmentList.Where(a => a.Item1 <= DateTime.Now);
            if (appointments.Count() > 0)
            { 
                string msg = "The following appointments are due:\n\r\n\r";

                // For this simple demo we are displaying all appointments that are due at or before the current time
                foreach (var item in appointments)
                    msg += String.Format("{0} - {1}\n\r\n\r", item.Item2, item.Item1);

                MessageBox.Show(msg);
            }
        }
    }
}</tuple></tuple>