[sticklord]

Hey, I haven't tested this or anything but its just a gut feeling. Swap the bottom part with this:

CastingBarFrame:HookScript('OnUpdate', function(self, elapsed)
    if not self.timer then return end
    if self.update and self.update < elapsed then
        if self.casting then
            self.timer:SetText(format("%.1f", max(self.maxValue - self.value, 0)))
        elseif self.channeling then
            self.timer:SetText(format("%.1f", max(self.value, 0)))
        else
            self.timer:SetText("")
        end
        self.update = 0.1
    else
        self.update = self.update - elapsed
    end
end)