just adding to the thread, the IRS does state that a day of rental below FMV is not a date of rental, but rather a day of personal use. So, in effect, each day is either a day of personal use or a day of rental - they are distinct and disjoint:
Left side: page 18:
https://www.irs.gov/pub/irs-pdf/p527.pdf
What is a day of personal use? A day of personal use of a dwelling unit is any day that the
unit is used by any of the following persons.
1. You or any other person who owns an interest in it, unless you rent it to another owner as their main home under a shared equity financing agreement (defined later). However, see Days used as a main home
before or after renting, later.
2. A member of your family or a member of the family of any other person who owns an interest in it, unless the family member uses the dwelling unit as their main home and pays a fair rental price. Family includes only your spouse, siblings, half siblings, ancestors (parents, grandparents, etc.), and lineal descendants (children,
grandchildren, etc.).
3. Anyone under an arrangement that lets you use some other dwelling unit.
4. Anyone at less than a fair rental price
