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

The Buddha’s bottom line:

Good men! Understand my sincere and infallible words by faith!

He said to the great multitude again, “Understand my sincere and infallible words by faith!”

He said to them once again, “Understand my sincere and infallible words by faith!”

And three times plus one the Bodhisattvas pleaded to learn the answer to yesterday’s questions.

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.

During those uncountable many years:

All this time I have been living in this Sahā-World, and teaching [the living beings of this world] by expounding the Dharma to them.

This Sahā-World

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. The living beings are various in their natures, desires, deeds, thoughts and opinions. Therefore, I expounded the dharma with various stories of previous lives, with various parables, similes and discourses, in order to cause all living beings to plant the roots of good. I have never stopped doing what I should do. As I said before, it is very long since I became the Buddha. The duration of my life is innumerable, asaṃkhya kalpas. I am always here. I shall never pass away.

And in gāthās:

The [perverted] people think:
“This world is in a great fire.
The end of the kalpa [of destruction] 11 is coming.”
In reality this 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 mandārava-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.”

THE PARABLE OF THE SKILLFUL PHYSICIAN AND HIS SICK CHILDREN

The Buddha said: “I am like the father. It is many hundreds of thousands of billions of nayutas of asaṃkhyas of kalpas since I became the Buddha. In order to save the [perverted] people, I say expediently, ‘I shall pass away.’ No one will accuse me of falsehood by the [common] law.

And in gāthās:

All of you, wise men!
Have no doubts about this!
Remove your doubts, have no more!
My words are true, not false.

The physician, who sent a man expediently
To tell his perverted sons
Of the death of their father in order to cure them,
Was not accused of falsehood although he was still alive.

In the same manner, I am the father of the world.
I am saving all living beings from suffering.

And most important:

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