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

Having last month heard of the expedient teachings, we learn why the Buddha says he will pass away.

“Good men! The duration of my life, which I obtained by the practice of the way of Bodhisattvas, has not yet expired. It is twice as long as the length of time as previously stated. Although I shall never enter into Nirvāṇa, I say to men of little virtue, ‘I shall pass away.’ I teach them with this expedient. Why is that? It is because, if they see me for a long time, they will not plant the roots of good, but become poor and base, and cling to the five desires so much that they will be caught in the nets of wrong views. If they think that I am always here, and do not think that I will pass away, they will become too arrogant and lazy to realize the difficulty of seeing me, and they will not respect me. Therefore I say [to them] expediently, ‘Bhikṣus, know this! It is difficult to see a Buddha who appears in [this] world.’ Why is that? It is because some men of little virtue cannot see me even during many hundreds of thousands of billions of kalpas while the others can. Therefore, I say [to them], ‘Bhikṣus! It is difficult to see a Tathāgata.’ Those who hear this and know that it is difficult to see me, will adore me, admire me, and plant the roots of good. Therefore l say [to them], ‘I shall pass away,’ although I shall not.

“Good men! All the Buddhas, all the Tathāgatas, do the same as I do. [They expound their teachings] for the purpose of saving all living beings. Therefore, [their teachings] are true, not false.

See The ‘Real World’