Based on entering a test case in TurboTax, the number of months that the office is used during the year does not factor in to the amount of mortgage interest or property tax that you enter. The number of months is used to calculate the home office deduction based on the Simplified Method only.
You should prorate the amounts for the mortgage interest and property tax before entering the amounts into the home office section of your Schedule C.
Take a look at your Form 8829 before filing your return to be sure that the calculations make sense for your situation. The mortgage interest and property tax deductions will be on lines 10 and 11. This is the number before calculating the portion applicable to the home office based on the square footage. That result is on line 13.
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