What's wrong with my TimeTable Countdown Script?

I am making a TimeTable where it Countsdown from the NumberValue to 0, but I want it to re-count it down back to 0 again, instead of -1, -2, e.t.c. I even made another NumberValue to restore it back to it's original number, but it doesn't work. Also, this is on a SurfaceGUI.

while true do
	script.Parent.Text = script.Parent.TValue.Value
	wait(1)
	script.Parent.TValue.Value = script.Parent.TValue.Value - 1
	
	if script.Parent.TValue == 0 then
		script.Parent.TValue = script.Parent.OValue - 1
		
		end
	end