Especially Part 4-It claims Buddhism influenced Upanishads and they are post-Buddhist and lists many contemporaries of Upanishadic Sages during Buddha's time. http://bps.lk/olib/wh/wh150_Joshi_Brahmanism-Buddhism-and-Hinduism.html#IVDateoftheOldestUpani7779ads
I have commented on some parts of this polemical article. Those parts are put in brackets. I have avoided commenting on the highly speculative part where different individuals have been discussed.
Opinion of early scholars
Using 19th and early 20th century scholars to date the Upanishads is hazardous. Some of these scholars believed that the universe started around 4000 BCE. Modern scholars are hesitant to precisely date the Upanishads. One said that dating the Upanishads accurately is like building a house of cards.
Archaic Language argument
[The only argument for placing the oldest Upaniṣads in the 6th century B. C. is the archaic character of their language. But their language can be compared only with the Mahābhārata and Rāmāyaṇa, which are very late composite compilations, or with the language of Pāṇini and the Bṛhad-devatā which have been placed in the fourth and third centuries BCE. There is thus no sound linguistic evidence to consider the Bṛhadāraṇyaka and Chāndogya Upaniṣads as pre-Buddhist in origin.]
The oldest Upanishads are written in archaic language compared to Mahabharata. That is accepted by every scholar. What does this tell us about the date for the Upanishads?
As we are certain there were multiple authors rather than a single one, the question of when the Mahabharata was composed is somewhat pointless. It wasn’t composed on a single date. It was composed over a span of more than 1000 years, perhaps between 800 BCE and 400 CE. It is impossible to be more accurate than that.
Mahabharata, Introduction by Bibek Debroy
So if the oldest Upanishads were composed before the oldest part of Mahabharata then they were composed before 800 BCE, long before Buddha.
About Tevijja Sutta
[The Tevijjā Sutta does not know the way of the Upaniṣads.]
I have posted a small excerpt from the Sutta.
‘Then you say, Vāseṭṭha, that the Bhikkhu is free from household and worldly cares, and that Brahmā is free from household and worldly cares. Is there then agreement and likeness between the Bhikkhu and Brahmā?’ ‘There is, Gotama! ‘Very good, Vāseṭṭha. Then in sooth, Vāseṭṭha, that the Bhikkhu who is free from household cares should after death, when the body is dissolved, become united with Brahmā, who is the same—such a condition of things is every way possible!’
Excerpt from Tevijja Sutta, Digha Nikaya
This seems to me pretty close to the Upanishads.
Quote from Brhadaranyaka Upanishad
[ In Bṛh 3.2.13 it is stated that after death the different parts of a person return to the different parts of Nature from whence they came, that even his soul (ātman) goes into space and that only his karma, or effect of work, remains over. This is a clear reflection of the Buddhist doctrine.]
A student of Yajnavalkya says that the self goes into Akash. Yajnavalkya does not say that. Moreover, they talk in secret and the Upanishad does not say that only his karma remains as claimed by the author.
“.. The two went away and deliberated. What they said was karman and what they praised was karman. Verily one becomes good by good action and bad by bad action.”
Brhadaranyaka Upanishad III.2.13
There is a similarity with Buddhist doctrine but not the same. In fact it is entirely possible that Buddhist doctrine was influenced by the Upanishad.
Katha Upanishad quote
[The Kaṭha Upaniṣad criticises the Buddhist doctrine of the plurality of elements (dharmas). It says, “Just as the water fallen over rocks is scattered and lost among the hills, likewise, he who holds the existence of separate dharmas is lost after them.” (Kaṭha Upaniṣad, IV. 14.). The term “dharma” in the phrase pṛthag-dharmān does not mean “quality” as Hume has translated. The theory of dharmas, or elements of mind and matter, was a Buddhist theory taught by the Buddha. The fact that the Kaṭha Upaniṣad is aware of it and criticises its expounders proves that this old Upaniṣad cannot be earlier than the fifth century BCE.]
This is an example of text torturing. This shloka does not say ‘he who holds the existence of separate dharmas’. This shloka is spoken by Yama in answer to Nachiketa’s question as to what happens to someone after death. What would be the point in criticizing Buddhist doctrine of dharma when answering as to what happens after death? Such an interpretation is completely out of context. What does this shloka actually say?
yathodakam durge vrstam parvatesu vidhavati evam dharman prthak pasyams tan evanuvidhavati
As rainwater falling on a mountain peak runs down the rocks in all directions, even so he who sees the attributes as different from Brahman verily runs after them in all directions.
Katha Upanishad II.i.14 ( and not IV.14 in the text)
The word dharma has been translated as attribute.
The word dharma in the text means, literally, attribute, which according to Advaita Vedanta, is non-different from substance.
The Upanishads translated by Swami Nikhilananda
This verse means that anyone who runs after the multiplicity of the relative world becomes a victim of ever recurring death and does not attain liberation. It has nothing to do with the fanciful claim that a Buddhist doctrine is being criticized. Even Katha Upanishad is older than Buddha.
Did Buddha attack the central concept of the Upanishads?
[Dr. Kane’s view that the Buddha and his early pupils did not attack the central Upaniṣad conception of the immanence of Brahmā is ill conceived. As a matter of fact, this conception of a neuter Brahman or absolute Ātman of the Upaniṣads had not come into vogue in the time of the Buddha. No Pāli Sutta refers to the theory of Upaniṣadic Brahman as the ultimate reality and the question of its criticism does not arise at all. ]
Just go through Tevijja Sutta and make up your mind about this claim.
Do we know for certain Buddha's date?
[The venerable antiquity of the older Upaniṣads is thus a matter of mere traditional belief. Scholars heretofore have been persuaded to believe that the Buddha’s teachings are partly presupposed by the older Upaniṣads. Our contention, however, is that the Upaniṣads have been greatly influenced by the Buddha’s teachings. The Buddha’s date (624–544 BCE) is certain; the date of the Upaniṣads, on the other hand, is a matter of traditional bias.]
There is no certainty about Buddha’s date. Buddha’s date was believed for most of 20th century to be c 560-480 BCE. Recently scholars have claimed the date to be 480-400 BCE. Actually Buddha's date is uncertain and not certain as claimed in this polemics.