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Home office deduction part of year

I am using the installed version of TurboTax Home and Business.   I am trying to enter information on a home office.   I used the office only for the first six months of the year, then retired.   Do I enter the expenses (mortgage interest, utilities, etc.) for the entire year or only prorate for the six months of the year when I used the office?    

When I entered the entire year's expenses, TurboTax askes how many months I used the office, but calculates the same deduction whether I enter 12 months or 6 months or even 1 month.