The law says at Section 280A(e):
In any case where a taxpayer who is an individual or an S corporation uses a dwelling unit for personal purposes on any day during the taxable year (whether or not he is treated under this section as using such unit as a residence), the amount deductible under this chapter with respect to expenses attributable to the rental of the unit (or portion thereof) for the taxable year shall not exceed an amount which bears the same relationship to such expenses as the number of days during each year that the unit (or portion thereof) is rented at a fair rental bears to the total number of days during such year that the unit (or portion thereof) is used.
https://www.law.cornell.edu/uscode/text/26/280A#e
