Elwell and Beitzel argue that since 4.18-22 records the genealogy of David, “The book must have been written sometime after the beginning of David’s reign.” Easton is matter-of-fact, “The author of this book was probably Samuel, according to Jewish tradition.” Specifically, it is the Talmud which says “Samuel wrote the books which bear his name and the book of Judges and Ruth” (B. Bat 14b-15a). Trible is more measured: “Though Jewish tradition assigned Ruth to the prophet Samuel, scholarship has remained properly silent on the subject. The author is unknown. Nevertheless, commentators have assumed a male gender for the storyteller, an assumption not unchallenged.”
As elusive as the author is, so too is the date. However, what one believes about the one is going to influence the other. Therefore, according to Trible, an exilic or postexilic date has been postulated due to “discrepancies with the Deuteronomic law.” On the other hand, “Many others, however, argue for a preexilic composition between the 10th and 7th centuries b.c.e. They detect linguistic features, classical prose, legal and theological perspectives that fit these earlier periods.”  Thus, with this earlier date, Samuel or some other contemporary could have penned the book. Like with the author, an exact date remains out of reach.
Most scholars agree that the book serves as an apology for the Davidic kingdom. Block argues that the purpose of the book is to authenticate the royal linage through a four act drama. He writes, “The author’s aim is to explain how, in the providence of God, the divinely chosen King David could emerge from the dark period of the judges.” Elwell and Beitzel agree and go further saying, “The book may be considered as a justification for including the godly Moabitess in the nation of Israel.”
 Ibid., 843.
 Elwell and Beitzel, 1871.