Matsya Purana — Measures of Time: Caturyuga Computation
त्रीणि वर्षसहस्राणि मानुषेण प्रमाणतः त्रिंशदन्यानि वर्षाणि स्मृतः सप्तर्षिवत्सरः //
trīṇi varṣasahasrāṇi mānuṣeṇa pramāṇataḥ triṃśadanyāni varṣāṇi smṛtaḥ saptarṣivatsaraḥ //
By human reckoning, three thousand years—together with another thirty years—are remembered as one ‘Saptarishi year’, the year measured by the cycle of the Seven Sages.
It does not describe Pralaya directly; it provides a technical unit of sacred chronology (the Saptarishi-year) used to structure long cosmic and historical time spans that frame cycles of creation and dissolution.
By defining an authoritative time-measure, it supports dharma in practice—ritual calendars, royal record-keeping, and the traditional dating of lineages and eras that kings preserve as part of righteous governance.
No Vāstu rule is stated, but such time-reckoning is commonly used to time rites, consecrations, and calendrical observances—background chronology that supports correct ritual scheduling.