Deduct work time

I know this would be a complicated function to implement but for me it would make it the perfect application. I have not found it in any software so far.

Instead of counting work time, time would be deducted from an amount assigned to a tag.

I use the labels as clients. Thus, client1 would have 2 hours of work and when the counter it's start, the time would be discounted until it reached 0.

Thanks.