Matsya Purana — Soma
तस्मात्पञ्चदशे सोमे कला वै नास्ति षोडशी तस्मात्सोमस्य विप्रोक्तः पञ्चदश्यां मया क्षयः //
tasmātpañcadaśe some kalā vai nāsti ṣoḍaśī tasmātsomasya viproktaḥ pañcadaśyāṃ mayā kṣayaḥ //
Therefore, on the fifteenth tithi for Soma (the Moon), the sixteenth kalā is indeed not present. Hence, O Brahmin, I have explained that the Moon’s waning (kṣaya) occurs on the fifteenth tithi.
This verse is not about cosmic Pralaya; it explains a smaller-scale doctrine of “kṣaya” (waning/decline) of Soma’s kalās within the lunar month, used for ritual timekeeping rather than world-dissolution.
By clarifying when the Moon’s kalā is considered diminished on the fifteenth tithi, it supports correct observance of vratas, dāna, and śrāddha scheduling—key household and royal duties in Purāṇic dharma that depend on accurate tithi-based timing.
The significance is ritual (not architectural): it provides a calendrical rule about Soma’s kalās and the fifteenth tithi, useful for determining proper timing for fasts, worship, and other rites aligned with lunar phases.