Day 21 covers all of Chapter 16, The Duration of the Life of the Tathāgata.
Having last month considered the Parable of the Skillful Physician and his Sick Children, we consider whether the Buddha can be accused of lying when he says he’ll enter into nirvana.
“Good men! What do you think of this? Do you think that anyone can accuse this excellent physician of falsehood?”
“No, World-Honored One!”
The Buddha said:
“I am like the father. It is many hundreds of thousands of billions of nayutas of asaṃkhyas 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.”