瑜伽師地論略纂 1.94

1.94

Regarding the kalpa periods: only the twenty kalpas of staying have increase and decrease, so twenty can be spoken of; since the three kalpas of formation, destruction, and emptiness have neither increase nor decrease, they are measured according to the duration of a kalpa of staying. In the lifespans of the first dhyāna, the "devas who benefit in front of Brahmā" are the second, the Brahmapurohita devas, because they perform beneficent service before the Great Brahmā King.