Datetime in a For loop

I have a for loop on a date that never counter

increases. I didn't see the reason. Can you help me?

private void ubGO_Click(object sender, EventArgs e)
{
    DateTime startDate = udteMin.DateTime.Date;
    DateTime endDate = udteMax.DateTime.Date;

    for (DateTime counter = startDate; counter <= endDate; counter.AddDays(1))
    {
        MessageBox.Show(counter.Date.ToString() + "            " + counter.AddDays(1).Date.ToString());
    }
}