The first day of the normal 2018 tax year is January 1, 2018. The last day of the normal 2018 tax year is December 31, 2018. The number of days between and including 1/1/2018 and 12/31/2018 is 365.
Based on the information you have provide, the number of days is 365. The days not rented in 2019 do not count.