Matsya Purana — Measures of Time: Caturyuga Computation
पित्र्ये रात्र्यहनी मासः प्रविभागस् तयोः पुनः कृष्णपक्षस् त्वहस्तेषां शुक्लः स्वप्नाय शर्वरी //
pitrye rātryahanī māsaḥ pravibhāgas tayoḥ punaḥ kṛṣṇapakṣas tvahasteṣāṃ śuklaḥ svapnāya śarvarī //
In the realm of the Pitṛs (ancestors), a month is made up of their day and night; and its division is as follows: the dark fortnight (kṛṣṇa-pakṣa) is their daytime, while the bright fortnight (śukla-pakṣa) is their night, appointed for sleep.
It does not describe Pralaya directly; it gives a cosmological time-reckoning where the Pitṛs experience the lunar fortnights as their day and night, a common Purāṇic model of differing temporal scales across realms.
It supports correct observance of śrāddha and ancestral rites by grounding them in Pitṛ-time: householders (and kings as patrons of dharma) should align offerings and ritual calendars with the lunar pakṣas as taught in the Purāṇa.
Ritually, it clarifies the pakṣa-based timing connected to Pitṛs—dark fortnight as their ‘day’ and bright fortnight as their ‘night’—useful for planning śrāddha/ancestral offerings; no Vāstu or temple-construction rule is stated in this verse.