Subtract Time

I want to display a timer that counts down to zero when application expires so user knows when task needs to be complete. I want to display this timer in “hh:mm:ss” format. I have added one hour to current time and when I try to subtract the current time from this new time, the code returns and empty set. Can someone help me with some VB code that can subtract one “hh:mm:ss” time from another to display a result in “hh:mm:ss” that I can then display on a label button.
-tia Kei