Section on the Duration of the Three Great Calamities
According to the *Abhidharma Mahāvibhāṣa Śāstra*: "There are three kinds of kalpas (eons): the intermediate kalpa, the kalpa of formation and destruction, and the great kalpa.
Intermediate kalpas are also of three types: the kalpa of decreasing, the kalpa of increasing, and the kalpa of both increasing and decreasing. In the kalpa of decreasing, human lifespan decreases from countless years down to ten years. In the kalpa of increasing, human lifespan increases from ten years up to eighty thousand years. In the kalpa of both increasing and decreasing, lifespan goes from increasing back to decreasing.
One kalpa of decreasing, one kalpa of increasing, and eighteen kalpas of both increasing and decreasing together make twenty intermediate kalpas. During these twenty intermediate kalpas, the world is formed. Then another twenty intermediate kalpas follow, during which the formed world remains stable. These two stages together are called the kalpa of formation. Then the world undergoes destruction for twenty intermediate kalpas, followed by a period of emptiness for another twenty intermediate kalpas. These two stages together are called the kalpa of destruction. In total, eighty intermediate kalpas are called a great kalpa."