how to subtract two txtbox datetime value

refer mahesh sir's blog here http://www.c-sharpcorner.com/UploadFile/mahesh/TwoDatesDifference08052005152233PM/TwoDatesDifference.aspx

try this logic ..

            DateTime christmas = Convert.ToDateTime(textBox1.Text);
            DateTime newYears = Convert.ToDateTime(textBox2.Text);
            TimeSpan span = newYears.Subtract(christmas);
            textBox3.Text = span.ToString();