Here is one possible solution:
private void UpdateTotalTime()
{
DateTime timeStart;
DateTime timeEnd;
if (DateTime.TryParse(textBox1.Text, out timeStart) && DateTime.TryParse(textBox2.Text, out timeEnd))
{
TimeSpan span = (timeEnd - timeStart);
textBox3.Text = span.ToString("c");
}
}
Just call this function when the text in your two input textboxes changes.