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

Day 21 Full Text

Having last time heard Śākyamuni’s call to “Understand my sincere and infallible words by faith,” he consider Śākyamuni’s 500-dust-atom lifetime.

Thereupon the World-Honored One, seeing that they repeated their appeal even after they repeated it three times, said to them:

“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, Śākyamuni Buddha, left the palace of the Śākyas, sat at the place of enlightenment not far from the City of Gayā, and attained Anuttara-samyak-saṃbodhi [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. Suppose someone smashed into dust five hundred thousand billion nayuta asaṃkhya worlds, which were each composed of one thousand million Sumeru-worlds, and went to the east [carrying the dust with him). When he reached a world at a distance of five hundred thousand billion nayuta asaṃkhya worlds [from this world], he put a particle of dust on that world. 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 asaṃkhya 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 Śrāvaka or Pratyekabuddha could count them even by his wisdom-without-āsravas. We are now in the state of avaivartika, but cannot, either. World-Honored 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 on hundred thousand billion nayuta asaṃkhyas larger than the number of the particles of the dust thus produced. All this time J have been living in this Sahā-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 asaṃkhya worlds outside this world.

The Introduction to the Lotus Sutra explains the 500-Dust-Atom Kalpas:

By means of these enormous numbers, numbers far beyond the ability of mathematics to conceive, Sakyamuni compares his life span to eternity. The simile which he uses is commonly called the “Five Hundred Dust-atom Kalpas.”