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

Having last month compared Śākyamuni to the Physician at the conclusion of the prose section, we repeat in gāthās.

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.

The Daily Dharma from May 21, 2017, offers this:

The Buddha sings these verses in Chapter Sixteen of the Lotus Sūtra. This is part of the explanation that his existence in the physical form of Śākyamuni was only one way that he leads all beings to enlightenment. The Buddha described his ever-present nature as the most difficult of his teachings to believe and understand. Recognizing the presence of the Buddha in the world is the same as recognizing Buddha nature in ourselves and all beings. Knowing that the Buddha is always available to help us in this difficult practice of the Wonderful Dharma means we do not need to rely on our own limited abilities. The store of the Dharma is always available to us. We just need to find it and use it.

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