What would be the "available date" for a house that was partially rented/residential for 3 months and converted to rental in the last 3 months. 6 months ownership in 21

Your best option is to enter this as two Rental Properties. This treatment allows you to enter different in-service dates (and rental days) for each unit and track income & expenses for each unit separately. 

The available date would be the date you listed it for rent, both for the mixed-use property and for the true rental. The mixed-use property would have an earlier available date.

Divide any shared expenses based on the rental days for each use. TurboTax doesn't handle this well, especially should you sell the property in the future. By segregating the income and expenses to each period, you will have good records on the rental activity.

You will need to enter the date you stopped using the mixed-use property as a rental. This isn't a sale, you simply ended the rental activity.