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

As it was then, so it is today: We must understand the Buddha’s “sincere and infallible words by faith.”

The Bodhisattvas, headed by Maitreya, agreed three times and once more for good measure.

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, Sakyarnuni 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.

And:

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.

The choice of teachings:

Good men! When some people came to me, I saw the strength of the power of their faith and of the other faculties of theirs with the eyes of the Buddha. Then I named myself differently, and told them of the duration of my life differently, according to their capacities. I also said to them, ‘I shall enter into Nirvana.’ I expounded the Wonderful Dharma with these various expedients, and caused the living beings to rejoice.

Good men! When I saw that some people of little virtue and of much defilement were seeking the teachings of the Lesser Vehicle, I told them, ‘I renounced my family when I was young, and attained Anuttara-samyak-sarnbodhi [forty and odd years ago].’ In reality I became the Buddha in the remotest past as I previously stated. I told them this as an expedient to teach them, to lead them into the Way to Buddhahood.

The Buddha’s perspective:

All that I say is true, not false, because I see the triple world as it is. I see that the triple world is the world in which the living beings have neither birth nor death, that is to say, do not appear or disappear, that it is the world in which I do not appear or from which I do not disappear, that it is not real or unreal, and that it is not as it seems or as it does not seem. I do not see the triple world in the same way as [the living beings of] the triple world do. I see all this clearly and infallibly.

PARABLE OF THE SKILLFUL PHYSICIAN AND HIS SICK CHILDREN

We are the Buddha’s delusional children:

He said to them, ‘Know this! Now I am old and decrepit. I shall die soon. I am leaving this good medicine here. Take it! Do not be afraid that you will not be cured!’

We take the medicine and wait to see the Buddha:

On hearing that they had recovered their health, the father returned home, and showed himself to them.

This Saha-World is the Pure World:

The [perverted] people think:
“This world is in a great fire.
The end of the kalpa [of destruction] is coming.”
In reality trus world of mine is peaceful.
It is filled with gods and men.

The gardens, forests and stately buildings
Are adorned with various treasures;
The jeweled trees have many flowers and fruits;
The living beings are enjoying themselves;
And the gods are beating heavenly drums,
Making various kinds of music,
And raining mandarava-flowers on the great multitude and me.

[This] pure world of mine is indestructible.
But the [perverted] people think:
“It is full of sorrow, fear, and other sufferings.
It will soon burn away.”

Because of their evil karmas,
These sinful people will not be able
To hear even the names of the Three Treasures
During asamkhya kalpas.

To those who have accumulated merits,
And who are gentle and upright,
And who see me living here,
Expounding the Dharma,
I say:
“The duration of my life is immeasurable.”
To those who see me after a long time,
I say, “It is difficult to see a Buddha.”

I can do all this by the power of my wisdom.
The light of my wisdom knows no bound.
The duration of my life is innumerable kalpas.
I obtained this longevity by ages of practices.

Underlying everything:

I am always thinking:
“How shall I cause all living beings
To enter into the unsurpassed Way
And quickly become Buddhas?”