Day 21 covers all of Chapter 16, The Duration of the Life of the Tathāgata.
This says it all:
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-Honor done, 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.’
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.
It is only by faith that any of this makes sense.
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.
Sakyamuni attained in enlightenment in this Saha World and has never left. He is still hear and forever will be. It is only as an expedient that we assume anything else.
Good men! All the sutras that I expounded [hitherto] were for the purpose of saving all living beings. I told the stories of my previous lives [in some sutras,] and the stories of the previous lives of other Buddhas [in other sutras]. I showed my replicas [in some sutras,] and my transformations [in other sutras]. I described my deeds [in some sutras,] and the deeds of others [in other sutras]. 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, asamkhya kalpas. I am always here. I shall never pass away.
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 asamkhyas 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 for me personally, the most important gatha in the entire Lotus Sutra:
I am always thinking:
‘How shall I cause all living beings
To enter into the unsurpassed Way
And quickly become Buddhas?’