Day 21 covers all of Chapter 16, The Duration of the Life of the Tathāgata.
Having last month considered the lesson of the Parable of the Skillful Physician and His Sick Children, we repeat in gāthās the many hundreds of thousands of billions of trillions of asaṃkhyas of kalpas since Śākyamuni became the Buddha.
Thereupon the World-Honored One, wishing to repeat what he had said, sang in gāthās:
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.Although I always live here
With the perverted people
I disappear from their eyes
By my supernatural powers.When they see me seemingly pass away,
And make offerings to my śarīras,
And adore me, admire me,
And become devout, upright and gentle,
And wish to see me
With all their hearts
At the cost of their lives,
I reappear on Mt. Sacred Eagle
With my Saṃgha,
And say to them:
“I always live here.
I shall never be extinct.
I show my extinction to you expediently
Although I never pass away.
I also expound the unsurpassed Dharma
To the living beings of the other worlds
If they respect me, believe me,
And wish to see me.
You have never heard this
Therefore, you thought that I pass away.”