Part of Buddhist doctrine involves believing that the cycle of Samsara has been occurring for a truly unfathomable length of time. Buddhist time is broken into periods called Kalpas, often translated as "eons" in English.
Buddha once described the length of an eon as follows:
Samyutta Nikaya - Anamataggasamyutta Sutta
5 At Sāvatthī. Then a certain bhikkhu approached the Blessed One, paid homage to him, sat down to one side, and said to him: "Venerable sir, how long is an aeon?" "An aeon is long, bhikkhu. It is not easy to count it and say it is so many years, or so many hundreds of years, or so many thousands of years, or so many hundreds of thousands of years."
"Then is it possible to give a simile, venerable sir?" "It is possible, bhikkhu," the Blessed One said. "Suppose, bhikkhu, there was a great stone mountain a yojana long, a yojana wide, and a yojana high, without holes or crevices, one solid mass of rock. At the end of every hundred years a man would stroke it once with a piece of Kāsi cloth. That great stone mountain might by this effort be worn away and eliminated but the aeon would still not have come to an end. So long is an aeon, bhikkhu. And of aeons of such length, we have wandered through so many aeons, so many hundreds of aeons, so many thousands of aeons, so many hundreds of thousands of aeons. For what reason? Because, bhikkhu, this saṃsāra is without discoverable beginning…. It is enough to be liberated from them."
Above, a "yojana" is around 5 kilometers. It would take a functionally infinite amount of time to wear down a solid rock in the shape of a cube 5 kilometers wide by brushing it once with a cloth every 100 years. Suppose that, optimistically, it takes 1,000 brushes to get through a cubic foot of rock. There are about 4.41 trillion cubic feet in Buddha's 5 km wide cubic mountain. So, being very optimistic, it would take:
Throughout his preaching, Buddha assumes that many such eons have passed. According to the Buddhavamsa of the Khuddaka Nikaya, he is said to have taught that the current eon has had three Buddhas before him, Kakusandha, Konagamana, and Kassapa:
Khuddaka Nikaya - Buddhavamsa 27
17 Thirty-one eons ago there were two leaders, Sikhīn and Vessabhū, unequalled, matchless.
18 In this Bhadda-eon there have been three leaders, Kakusandha, Koṇāgamana and the leader Kassapa.
19 I at the present time am the Self-Awakened One, and there will be Metteyya. These are the five Buddhas, wise ones, compassionate toward the world.
20 When these kings under Dhamma had pointed out the Way for countless crores of others, they waned out with their disciples.
Notice that prior to this eon, Buddha said that there was a Buddha 31 eons ago. This is one of many such massive gaps in there being a Buddha that he recounts in this chronicle. For example, he says that there were 70,000 eons between Padumuttara Buddha and Sumedha Buddha. It seems that Buddha wanted people to believe that someone like him only came around once in an unfathomably long period of time, and for some reason, certain eons were apparently deprived of any Buddha whatsoever.
In the Buddhavamsa, Buddha gives a brief biography of the three previous Buddhas of this current eon, wherein he inserts himself in past lives into their stories.
In these biographies, he says that Kakusandha's "life-span was forty thousand years", that he "was forty ratanas (feet?) tall. Golden lustre streamed forth for ten yojanas all round", and that he "awakened thirty thousand crores of devas and men", which is equal to 300 billion (Buddhavamsa 23). He says that Konagamana's "life-span was thirty thousand years", that he was "thirty cubits in height", and that Buddha himself, in a past life, "gave the teacher and the disciples silk from Pattunna, silk from China, Kasi silk" (Buddhavamsa 24). He says that Kassapa's "life-span was twenty thousand years", that he was "twenty ratanas in height", that Buddha himself, in a past life, "was then the brahman youth Jotipala, a famous repeater, expert in the mantras, master of the three Vedas" when he interacted with him, and that after he died, "a Conquerer's (Stupa)" was built for him "to the height of a yojana" (Buddhavamsa 25).
Here, Buddha speaks of developed civilizations like China existing long before they actually did, and gives an antiquity to the "three Vedas" which is likewise completely wrong, historically speaking. The absurdity of the lifespans, heights, and descriptions of civilizations in these biographies reflects a deep ignorance of scientific and historical reality.
Similar teachings can be found in the Digha Nikaya's Mahapadana Sutta:
Digha Nikaya - Mahapadana Sutta 1
4 'Monks, ninety-one aeons ago the Lord, the Arahant, the fully-enlightened Buddha Vipassi arose in the world. Thirtyone aeons ago the Lord Buddha Sikhī arose; in the same thirty-first aeon before this Lord Buddha Vessabhū arose. And in this present fortunate aeon the Lords Buddhas Kakusandha, Koṇāgamana and Kassapa arose in the world. And, monks, in this present fortunate aeon I too have now arisen in the world as a fully-enlightened Buddha.
5 The Lord Buddha Vipassi was born of Khattiya race, and arose in a Khattiya family; the Lord Buddha Sikhī likewise; the Lord Buddha Vessabhū likewise; the Lord Buddha Kakusandha was born of Brahmin race, and arose in a Brahmin family; the Lord Buddha Koṇāgamana likewise; the Lord Buddha Kassapa likewise; and I, monks, who am now the Arahant and fully-enlightened Buddha, was born of Kshatriya race, and arose in a Khattiya family.
6 The Lord Buddha Vipassī was of the Kondañña clan; the Lord Buddha Sikhī likewise; the Lord Buddha Vessabhū likewise; the Lord Buddha Kakusandha was of the Kassapa clan; the Lord Buddha Koṇāgamana likewise; the Lord Buddha Kassapa likewise; I who am now the Arahant and fully-enlightened Buddha, am of the Gotama clan.
7 In the time of the Lord Buddha Vipassī the life-span was eighty thousand years; in the time of the Lord Buddha Sikhī seventy thousand; in the time of the Lord Buddha Vessabhū sixty thousand; in the time of the Lord Buddha Kakusandhu forty thousand; in the time of the Lord Buddha Koṇāgamana thirty thousand; in the time of the Lord Buddha Kassapa it was twenty thousand years. In my time the life-span is short, limited and quick to pass: it is seldom that anybody lives to be a hundred.
Notice that above, some of the Buddhas are spoken of as being from the "Kshatriya" (Khattiya) race, and Kakusandha, Konagamana, and Kassapa are spoken of as being "born of Brahmin race". These are Caste categories which, though they existed at the time of Buddha, are incredibly anachronistic if being spoken of in the context of something that was supposedly taking place tens of thousands or hundreds of thousands of years ago. It would have made sense to Buddha that the way things were in his society was the way things always had been. However, historically speaking, these categories from Vedic-era India definitely did not exist at the times he is describing them.
Therefore, if these sayings do in fact go back to Buddha, then he was a deeply ignorant man, who invented stories to deceive gullible people, who could not verify what we can verify in modern times, namely, that what he was referring to is historically and scientifically impossible.
If, on the other hand, these sayings were invented by the earliest followers of Buddha, and enshrined wrongfully in the Pali Canon, then the earliest Buddhist communities saw it fit to prolifically fabricate the words of their lord, and these fabrications would come to have tremendous influence on the entire trajectory of the Buddhist religion, which views these texts as authentic and authoritative.