For Form 8824 (Like Kind Exchanges, IRC Section 1031), Line 3, you should use the date on which you originally acquired (i.e., bought) the property in the first instance. This should be the date of the purchase transaction recorded by your title company or attorney in the local county property records -- not the date upon which you later "converted" the property from a personal residence into a rental.
Thank you for asking about this clarification.