Не запускается таймер - C#

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 WindowsFormsApplication3
{
    public partial class Form1 : Form
    {
        private DateTime t1;
        private DateTime t2;

        public Form1()
        {
            InitializeComponent();
            //Setting numericUpDown1
            numericUpDown1.Maximum = 59;
            numericUpDown1.Minimum = 0;
            numericUpDown1.TabStop = false;
            //Setting numericUpDown2
            numericUpDown2.Maximum = 59;
            numericUpDown2.Minimum = 0;
            numericUpDown2.TabStop = false;
            //Button Start/Stop is unavalible
            button1.Enabled = false;
 
        }
        private void numericUpDown1_ValueChanged(object sender, EventArgs e)
        {
            if ((numericUpDown1.Value == 0) && (numericUpDown2.Value == 0))
                button1.Enabled = false;
            else
                button1.Enabled = true;
        }
 
        private void button1_Click(object sender, EventArgs e)
        {
            if(!timer1.Enabled)
            {
                t1 = new DateTime(DateTime.Now.Year, DateTime.Now.Month, DateTime.Now.Day);
                t2 = t1.AddMinutes((double)numericUpDown1.Value);
                t2 = t2.AddSeconds((double)numericUpDown2.Value);
                groupBox1.Enabled = false;
                button1.Text = "Stop";
 
                if(t2.Second < 9)
                    label1.Text += "0" + t2.Second.ToString();
                else
                    label1.Text += t2.Second.ToString();
                timer1.Interval = 1000;
                timer1.Enabled = true;
                groupBox1.Visible = false;
            }
            else
            {
                timer1.Enabled = false;
                button1.Text = "Start";
                groupBox1.Visible = true;
                numericUpDown1.Value = t2.Minute;
                numericUpDown2.Value = t2.Second;
            }
        }
 
        private void timer1_Tick(object sender, EventArgs e)
        {
            t2 = t2.AddSeconds(-1);
 
            if (t2.Minute < 9)
                label1.Text += "0" + t2.Minute.ToString() + ":";
            else
                label1.Text += t2.Minute.ToString() + ":";
 
            if (t2.Second < 9)
                label1.Text += "0" + t2.Second.ToString();
            else
                label1.Text += t2.Second.ToString();
            if (Equals(t1, t2))
            {
                timer1.Enabled = false;
                MessageBox.Show("Time out :)", "Timer", MessageBoxButtons.OK, MessageBoxIcon.Information);
 
                button1.Text = "Start";
                groupBox1.Enabled = true;
                numericUpDown1.Value = 0;
                numericUpDown2.Value = 0;
            }

        }
 
    }
 
}