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How to reduce nr. of available rooms by 1


i need to know how i can reduce the number of available rooms by 1.

I am renewing one room and it will be not available for a longer period.

I thought that i earlier have seen a function which would let me enter a time period and and a nr. of rooms that then will be added or decreased to the nr. of actually available rooms.

Anyone knows where i can find this again?