DateTime productLaunchDateTime = DateTime.Parse("1/12/2009 12:00:01 AM");
DateTime startDate = DateTime.Now;

//Calculate countdown timer.
TimeSpan t = productLaunchDateTime - startDate;
string countDown = string.Format("{0} Days, {1} Hours, {2} Minutes, {3} Seconds til launch.", t.Days, t.Hours, t.Minutes, t.Seconds);

This code shows for how long would be the Subject visible.

How would I do the code for a particular subject that it would dissapear from the listBox (where this subject is shown) when the count down would come to an end?