Depreciation - start date for depreciation of rental property

You should use the date the rental property is available to be rented. It does not start the day the first tenant moves in.
Generally, residential rental properties use a straight line 27.5 year life with a midmonth convention and nonresidential real properties use a straight line 39 year life with a midmonth convention to determine the depreciation deduction that is reported on IRS Form 4562.
The midmonth convention assumes the property was placed in service in the middle of the month and is built into the IRS depreciation table.