Hello and welcome to the forum
I would set up the availability of your resources like a bank account, so you make deposits and withdrawals on certain days. So a booking looks like the withdrawal a quantity of resource on a certain day, and then a deposit of that same quantity of resource again on the end of the booking.
Then, just like a bank balance, you sum the deposits and withdrawals up to the day you want to check availability for. That is the "balance" of resources in that "account." In the case of real-world items, of course, you can never have a negative balance in this account.