FORM1为主窗体，FROM2为子窗体，FROM2中需要调用FROM1中的DATATIMPICKER1的值

首先你将DATATIMPICKER1的modifiers属性设为public

在form1窗体中

  private void button2_Click(object sender, EventArgs e)
        {
           
            FROM2  f2 = new FROM2(this);
            f2.Show();
        }


在FROM2添加一个构造函数

  public FROM2  ()
        {
            InitializeComponent();
        }
        public FORM1  f1;
        public FROM2(FORM1 fm)
        {
            InitializeComponent();
            f1 = fm;
        }

        private void button1_Click(object sender, EventArgs e)
        {
            MessageBox.Show(f1.dateTimePicker1.Value.ToString());
        }