APA - https://owl.english.purdue.edu/owl/resource/560/01/
You are correct! If there is no known date for the source, the placeholder "n.d." should be inserted in both the in-text citation and on the reference page. This is to indicate that the publication date is not available or cannot be determined. Using "n.d." helps to maintain the proper formatting and citation style, and it allows readers to understand that the source does not have a specific date associated with it.