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

So I’d like to just reprint the whole thing. Forced to forgo that, I’d start by pointing out the Buddha’s repeated warning:

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!”

Is there any other way to believe than by faith?

“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.”

Continuing . . .

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.

The reality of the Triple World:

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. 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:

I know who is practicing the Way and who is not.
Therefore I expound various teachings
To all living beings
According to their capacities.

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