Day 21 covers all of Chapter 16, The Duration of the Life of the Tathāgata.
Having last month considered how long it has been since Śākyamuni became a Buddha, we consider the purpose of the various sutras.
“Good men! During this time I gave various names to myself, for instance, the Burning-Light Buddha. I also said, ‘That Buddha entered into Nirvāṇa.’ I did all these things only as expedients.
“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 Nirvāṇa.’ 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-saṃbodhi [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.
“Good men! All the sūtras that I expounded [hitherto] were for the purpose of saving all living beings. I told the stories of my previous lives [in some sūtras,] and the stories of the previous lives of other Buddhas [in other sūtras]. I showed my replicas [in some sūtras,] and my transformations [in other sūtras]. I described my deeds [in some sūtras,] and the deeds of others [in other sūtras]. 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 l 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.