Day 21 covers all of Chapter 16, The Duration of the Life of the Tathāgata.

After focusing on the need for “faith” and what is so difficult to believe, I want to step back at this point and just marvel at how long the Buddha has been the Buddha.

Listen to me attentively! I will tell you about my hidden core and supernatural powers. The gods, men and asuras in the world think that I, Sakyamuni Buddha, left the palace of the Sakyas, sat at the place of enlightenment not far from the City of Gaya, and attained Anuttara-samyak-sambodhi [forty and odd years ago]. To tell the truth, good men, it is many hundreds of thousands of billions of nayutas of kalpas since I became the Buddha.

Pausing here to underline hundreds of thousands of billions of nayutas. That’s hundreds of thousands of billions times a hundred thousand million. And then you multiply that by the number of years in a kalpa, which some estimate to be around 4.32 billion years.

Suppose someone smashed into dust five hundred thousand billion nayuta asamkhya worlds, which were each composed of one thousand million Sumeru-worlds, and went to the east [carrying the dust with them). When he reached a world at a distance of five hundred thousand billion nayuta asamkhya worlds [from this world], he put a particle of dust on that world.

Pausing again to underline a distance of five hundred thousand billion nayuta asamkhya worlds. That’s five hundred thousand billion times a hundred thousand million times 10 to the 52nd power. And that’s just the number of worlds between one particle of dust and the next.

Then he went on again to the east, and repeated the putting of a particle of the dust [on the world at every distance of five hundred thousand billion nayuta asamkhya worlds] until the particles of the dust were exhausted. Good men! What do you think of this? Do you think that the number of the world he went through is conceivable, countable, or not?

Maitreya Bodhisattva and others said to the Buddha:

“World-Honored One! Those worlds are innumerable, uncountable, inconceivable. No Sravaka or Pratyekabuddha could count them even by his wisdom-without-asravas. We are now in the state of avaivartika, but cannot, either. World-Honoured One! Those worlds are innumerable.

Thereupon the Buddha said to the great multitude of Bodhisattvas:

Good Men! Now I will tell you clearly. Suppose those worlds, whether they were marked with the particles of the dust or not, were smashed into dust. The number of the kalpas which have elapsed since I became the Buddha is one hundred thousand billion nayuta asamkhyas larger than the number of the particles of the dust thus produced. All this time I have been living in this Saha-World, and teaching [the living beings of this world] by expounding the Dharma to them. I also have been leading and benefiting the living beings of one hundred thousand billion nayuta asamkhya worlds outside this world.

And later he adds:

Good men! The duration of my life, which I obtained by the practice of the way of Bodhisattvas, has not yet expired. It is twice as long as the length of time as previously stated.

Eternity is a matter of perspective. For example, a quick Google search finds that the shortest lifespan of an animal is the Dolania americana, or mayfly. The adult females of the species live for less than five minutes. My single lifetime of more than 64 years encompasses something like 6,727,680 generations of female mayflies. Now consider that homo sapiens have been walking this earth for perhaps 10,000 generations.