Day 21 covers all of Chapter 16, The Duration of the Life of the Tathāgata.
Having last month considered whether the excellent physician was guilty of a falsehood, we consider the expediency of the Buddha’s Nirvāṇa.
Thereupon the World-Honored One, wishing to repeat what he had said, sang in gāthās:[1, 2]
It is many hundreds of thousands
Of billions of trillions
Of asaṃkhyas of kalpas
Since I became the Buddha.For the past innumerable kalpas
I have always been expounding the Dharma
To many hundreds of millions of living beings
In order to lead them into the Way to Buddhahood.In order to save the [perverted] people,
I expediently show my Nirvāṇa to them.
In reality I shall never pass away.
I always live here and expound the Dharma.