Reverse of an Amount and counting
You could still use counting if it's clear which direction you are actually counting in.
How long do you have left to retirement?
Down to fifty days and counting.
You can use and counting if the direction in which you count is clear from your context.
Otherwise, you can use counting down.
You could also use decreasing
50 days and decreasing
de·crease (dĭ-krēs′) To grow or cause to grow gradually less or smaller, as in number, amount, or intensity.