Matsya Purana — Soma
प्रकृतिः कृष्णपक्षस्य काले ऽतीते ऽपराह्णिके सायाह्ने प्रतिपद्येष स कालः पौर्णमासिकः //
prakṛtiḥ kṛṣṇapakṣasya kāle 'tīte 'parāhṇike sāyāhne pratipadyeṣa sa kālaḥ paurṇamāsikaḥ //
When the natural (proper) time of the dark fortnight (kṛṣṇa-pakṣa) has passed—i.e., when the afternoon (aparāhṇa) has elapsed—and Pratipad (the first lunar day) occurs in the evening, that time is to be classified as belonging to the Paurṇamāsika (full-moon-based) reckoning.
This verse is not about Pralaya; it addresses dharmic time-reckoning—how to classify a boundary case of lunar days (tithi) for ritual and calendrical purposes.
It supports correct observance of rites (vrata, śrāddha, yajña) by teaching how to decide the proper calendrical system when tithi transitions occur late in the day—an essential duty for householders and for kings who uphold public ritual order.
Ritually, it defines a Paurṇamāsika (full-moon-based) classification when Pratipad arises in the evening after the afternoon has passed, guiding accurate scheduling of monthly observances tied to lunar fortnight boundaries.