Datta, Manmathanatha, [Manmatha Nath Dutt], (1895). A prose English translation of the Mahabharata (tr. literally from the original Sanskrit text), Adi Parva, Mahabharata 1.2.377-378, Printed by H.C. Dass, Calcutta, p. 21: "There are Harivansa and Vavisya in its appendix. The number of slokas composed by the great Rishi in the Harivansa, is twelve thousand. These are the contents of the chapters called Parva Sangraha in the Bharata."
Lorenz, Ekkehard, (2007). "The Harivamsa: The Dynasty of Krishna", in: Edwin F. Bryant (ed.), Krishna: A Sourcebook, Oxford University Press, p. 97: "Sivaprasad Bhattacharyya, who holds that already Ashvaghosha (first century C.E.) referred to the HV, finds internal and external evidence that 'the H.V. was certainly an authoritative text by the first century A.C. and its later redaction took place about the end of the second or beginning of the third century A.C.'"
books.google.com
Debroy, Bibeck, (2016). Harivamsha, Penguin Books India, Gurgaon, "Introduction": "There are thus 5,965 shlokas in all of Harivamsha. Non-Critical versions will often have doubled this number, reflective of the slashing."
Hazra, Rajendra Chandra, (1975)[1940]. Studies in the Purāṇic Records on Hindu Rites and Customs, Motilal Banarsidass, p. 55: "[V]isnu-p. is earlier than the Bhagavata [...] Harivamsa also [...] being dated about 400 A. D. [...] Thus the date of composition of the Bhagavata falls in the sixth century A. D."
Hopkins, Washburn E., (2020) [1915]. "Date of Epic Poetry", in: E. Washburn Hopkins, Epic Mythology, p. 1: "The Mahabharata has been increased by the late addition of the Harivamsha (perhaps 200 A. D.) [...]"
Hopkins, Washburn E., (2020) [1915]. "Date of Epic Poetry", in: E. Washburn Hopkins, Epic Mythology, p. 2: "Northern version [of the Mahabharata] contains 84,126 verses, which, with the 16,375(526) verses of the Harivamsha, make 100,501(651) verses [...] It is therefore doubtful whether the attribution of a lakh of verses [in the Mahabharata] necessarily implies the existence, as part of the lakh, of the Harivamsha. Yet on the whole this is probable, owing to the fact that the expansion in [the Southern recension] appears for the most part to be due rather to the inclusion of new material than to the retention of old passages."
Hein, Norvin, (May, 1986). "A Revolution in Kṛṣṇaism: The Cult of Gopāla", in: History of Religions, Vol. 25, No. 4, Religion and Change: ASSR Anniversary Volume, The University of Chicago Press, p. 296.
pradipbhattacharya.com
Bhattacharya, Pradip, (May 27, 2017). "Review of André Couture: Krsna in the Harivamsha, Vol. 1, 2015": "Couture finds no cogent basis for Vaidya's dating of the HV to 300 or 400 CE. The recent conclusions of scholars like Hiltebeitel, Bailey, Sutton, Biardeau and Fitzgerald that the MB was compiled between 200 BCE and 200 CE as a response to Buddhism, would apply equally to the HV."
Bhattacharya, Pradip, (May 27, 2017). "Review of André Couture: Krsna in the Harivamsha, Vol. 1, 2015": "[C]outure conclude[s] that the Mathura described in the HV is evocative of cities of the Kushana era (1st to mid-3rd century CE) and not of the end of the Dvapara Yuga [...]"
The Mahabharata in Sanskrit: Book I: Chapter 2 in sacred-texts.com website, (MBh.1.2.69): "hari vaṃśas tataḥ parva purāṇaṃ khila saṃjñitam bhaviṣyat parva cāpy uktaṃ khileṣv evādbhutaṃ mahat." ["Hari Vamsa Purana known as Khila (supplement) and Bhavishya Parva also spoken as Khila are wonderful and great"].