Day 21 covers all of Chapter 16, The Duration of the Life of the Tathāgata.
Having last month considered the duration of time since Śākyamuni became the Buddha, we consider why the Buddha gave himself various names.
“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. l 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.