How to show days remaining

[oliboi]

I have an Excel document where I am inserting information about booking certain rooms in a building. We have a certain amount of days available that we can book rooms. For example, we are permitted only 35 weekdays room bookings rentals dates.

I want to show on my Excel document that anytime I fill in a row with a new booking date information, there is a cell with a number that always goes down with the remaining number of rental days available. I am wondering what is the best way to show this.