The "days rented" count starts from the first day a renter "could" have moved in.
The "days personal use" is asking the number of days you used the portion classified as a rental for personal use *AFTER* the date you converted it to a rental. What you used it for prior to converting it to a rental does not count for anything. Typically this will be ZERO days of personal use.
If you converted a percentage from personal use to rental use in the tax year, then you are also asked a question that is in fact *the wrong question* for your specific and explicit situation. The question reads:
"Percentage of time I used this item in this business in 2020 (e.g.;80%)"
That question (only for your specific situation if the conversion was done in 2020) should read "percentage of floor space I used in this business in 2020". So on that screen (the one right before the summary screen) enter the percentage of floor space that was classified as a rental for any period of time in 2020.
The program already knows how many days it was a rental, based on the date you entered for the in service date. But it doesn't know how many days of the rental period you may have used that space for personal use, and it doesn't know what percentage of the floor space was exclusive to the rental portion. (Even though the program did ask for that on an earlier screen, it doesn't use that number in the key math computation where it needs to use it.)
Note also that if done correctly, the program will only split mortgage interest and property taxes betweeen the SCH A and SCH E. It's up to you to prorate the property insurance and all other expenses. The program will only prorate those "other expenses" further, only if you have personal use days of the floor space, after the date you converted that space to rental.
