According to Pande, T.W. Rhys Davids places the chronology of the nikayas midway between the parnibbana and Ashoka, which accepting the modern date for the parinibbana of circa 400 BCE would place the composition of the nikayas about 325 BCE, merely 75 years after the death of the Buddha, but after the Parayana Vagga, Octades (Atthakka Vagga), and Patimokkha. The Atthakavadda and Parayanavagga may be found in the Khuddaka Pitaka, as the fourth and fifth sections of the Suttanipata. The pattimokkha is of course a summation of the Vinaya. Thus, these appear to be the oldest parts of the Pali Canon. (G.C. Pande, Studies in the Origins of Buddhism, 5th ed., Delhi: Motilal Banarsidass 2006, p. 20.)
Gombrich suggests that the nikayas are mid-4th century BCE. Rupert Gethin criticizes this view and suggests that they are somewhat later, "a few generations" after the parinibbana. ("Gethin on Gombrich: What the Buddha Thought")
According to noted University of Toronto Buddhologist A.K. Warder "the order of the five 'traditions' [referring to the agamas, a.k.a. nikayas] happens also to be the order of their authenticity." The first of these is the Digha Nikaya (Skt. Dirgha Agama). Warder claims that this is proved by "comparing the various available recensions." B.C. Law divides the Digha in half, placing the first part earlier than the second and third parts, which postdate the Four Great Nikayas, and distinguishes between an earlier pattimokkha of 152 rules and a later pattimokkha of 227 rules. Warder also considers the Ksudraka or Minor Tradition to be the least authentic portion of the Canon. At the same time he recognizes the Khadgavisanaghata, Munigatha, Sailagatha, Arthavargiyani Sutrani, Parayana, Sthaviragatha, Sthavirigatha, and Ityukta to constitute the "original nucleus of the Ksudraka common to all the schools," whereas the Udanas, Jatakas, and Avadanas "seem at least in origin to have been nothing but anthologies from the Tripitaka." (Warder, Indian Buddhism, 3rd rev. ed., Delhi: Motilal Banarsidass, 2000, pp. 196ff.).