Matsya Purana — Soma
चन्द्रभूर्यव्यतीपाते समे वै पूर्णिमे उभे प्रतिपत्प्रतिपन्नस्तु पर्वकालो द्विमात्रकः //
candrabhūryavyatīpāte same vai pūrṇime ubhe pratipatpratipannastu parvakālo dvimātrakaḥ //
When the lunar and solar motions are in equal measure at the Vyatīpāta (astronomical junction), and on both full-moon observances, the parva-period is to be understood as extending for two muhūrtas, from the moment the Pratipad (first lunar day) has begun.
It does not address Pralaya; it gives a technical dharma rule for calculating a sacred junction (parva) time based on lunar-solar conditions such as Vyatīpāta and the transition into Pratipad.
It guides correct observance of ritual timing—important for householders performing vrata, śrāddha, and festival rites, and for kings who must ensure public rites and calendrical observances are performed at the proper parvakāla.
Ritually, it defines the parva window as two muhūrtas beginning with the onset of Pratipad under specified lunar-solar conditions, which affects when fasting, offerings, bathing, and other parva-linked rites should be initiated.