How to undo this step

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C#

C#6.0

C#7.0

Actually when I check this button it inserts Date and time at the top of rich text box which is fine till here but now I want that when I click again on this button and I uncheck it, it should remove the date and time it inserted when I checked it

what happens till now is when I check the button it inserts date and time at top and when I uncheck it nothing happens but when I check it again it again inserts date and time at top
I've also tried using the checked state changed event but it clears the whole rich text box.

here is what I've done for this

What I have tried:

private RichTextBox Getnew()
{
    RichTextBox box = null;
    TabPage z = tabControl1.SelectedTab;
    if (z != null | z == null)
    {
        box = z.Controls[0] as RichTextBox;
    }
    return box;
}

private void dateTimeToolStripMenuItem_Click(object sender, EventArgs e)
{
    if (dateTimeToolStripMenuItem.Checked == true)
    {
        Getnew().Text = Getnew().Text.Insert(0, DateTime.Now.ToLongDateString() + DateTime.Now.ToLongTimeString()+"\n");
    }
}

private void dateTimeToolStripMenuItem_CheckStateChanged(object sender, EventArgs e)
{
    // Sorry I've Removed this.
}