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

By faith:

Thereupon the Buddha said to the great multitude including Bodhisattvas and others, “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!”

Thereupon the great multitude of Bodhisattvas, headed by Maitreya, joined their hands together and said to the Buddha, “World-Honored one, tell us! We will receive your words by faith.”

They said this three times. Then they said once again, “Tell us! We will receive your words by faith.”

After explaining just all how inconceivably long it has been since becoming the Buddha, Śākyamuni explains:

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. I also have been leading and benefiting the living beings of one hundred thousand billion nayuta asaṃkhya worlds outside this world.

And he’ll be around for awhile more:

The duration of my life is innumerable, asaṃkhya kalpas. I am always here. I shall never pass away.

(Parable of the Skillful Physician and His Sick Children)

This Triple World as it is:

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.

And in gāthās:

The [perverted] people think:
“This world is in a great fire.
The end of the kalpa [of destruction] 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 Buddha’s constant thought:

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