Days rented: The day count starts on the first day a renter "could" have moved in. Vacant periods between renters also count as days rented, provided you did not live in the property for one single day during that vacant period as your primary residence, 2nd home, or any other type of "personal pleasure" use.
So if you had a case of where a tenant moved out on May 1 of the tax year and you did not get a new tenant in until Aug 1 of that same tax year, then your days rented is "THE WHOLE YEAR", unless you lived in the property for one single day as your primary residence, 2nd home or any other type of "personal pleasure" use during that period of vacancy between tenants.
Days of Personal Use: The number of days you lived in the property as your primary residence, 2nd home or any other type of "personal pleasure" use WHILE THE PROPERTY WAS CLASSIFIED AS A RENTAL. What you did with the property "before" it was converted to a rental *does* *not* *count* for days of personal use.
Percentage of business use: The percentage of time the property was used in the business (which includes vacant periods between renters) starting from the date the property was converted to a rental. The time before it was converted to a rental *does* *not* *count* for this. So if your days of personal use is zero, then your percentage of business use is 100% (ONE HUNDRED PERCENT)
Days rented has no effect on depreciation
Days of personal use and percentage of business use *DOES* have an effect on depreciation.
