Schaff, Philip. "Ch. 27: Time of Composition." The Oldest Church Manual Called the Teaching of the Twelve Apostles. Second Edition, Revised and Enlarged. New York: Funk & Wagnalls, 1886, 119-123.
Schaff considers the Didache to be "one of the oldest, if not the very oldest, of the post Apostolic writings" (Schaff 1886, 119). He draws this conclusion based on the absence of any hint of a canon of Scripture, baptismal creed, dogmatic formulations, or mentions of heresy. The church shows all the signs of being in its infancy (Schaff 1886, 120).
Schaff observes that the Didache is quoted by Clement of Alexandria, ca. 200, does not have the doctrinal or ecclesiastical development of Irenaeus (ca. 180) or Justin Martyr (ca. 140), and seems to be summarized in a confused way by the Epistle of Barnabas (ca. 120 or 100) (Schaff 1886, 121). The Shepherd of Hermas makes enlargements of some parts of the Didache, and was perhaps written as early as ca. 92-100. It does not have the distinctions in offices found in Ignatius, c. 100-125. However, chapter 16 makes no suggestion of the destruction of Jerusalem. Schaff takes the entire text to be a summary of apostolic teaching which would have come after their time, so at least in the 90s (Schaff 1886, 122).