The p[roperty becomes a rental property when it is ready and available to be rented. If that date was the 20th, then a small percentage of what you paid can be allocated to that useage.
You place property in service in a rental activity when it is ready and available for a specific use in that activity. Even if you aren’t using the property, it is in service when it is ready and available for its specific use.
You will start with the lower of the fair market value vs the adjusted basis for converted property.
When a personal residence is converted to business use (or for use in the production of income), its starting point for basis for depreciation is the lower of
(1) the adjusted basis on the date of the conversion or
(2) the property's FMV at the time of the conversion.
