Countdown Timer

I can't figure out how to make a timer countdown rather than counting up, I know that it has to do with System::DateTime::Subtract. I have a DateTimePicker, and I want the timer to start right when you change the value of the DateTimePicker, and I want the countdown to be counting down from the value I choose. For example if I choose August  14, 2006 on the dateTimePicker, I want the timer to be counting down from that date.