Matsya Purana — Measures of Time: Caturyuga Computation
मन्वन्तरस्य संख्या तु मानुषेण निबोधत एकत्रिंशत्तथा कोट्यः संख्याताः संख्यया द्विजैः //
manvantarasya saṃkhyā tu mānuṣeṇa nibodhata ekatriṃśattathā koṭyaḥ saṃkhyātāḥ saṃkhyayā dvijaiḥ //
Now understand the measure of a Manvantara in human terms: the twice-born (dvija) have reckoned it, by established computation, as thirty-one crores of years.
It fixes the vast cosmic timetable by stating the human-year measure of a Manvantara, a key unit used to map cycles that culminate in larger dissolutions (pralayas) over greater spans.
By placing human life and governance within immense cosmic cycles, it encourages dharmic rule and disciplined household life with awareness of time, impermanence, and long-term moral order (dharma) across ages.
No direct Vāstu or ritual rule is stated; the significance is chronological—such time-units are used to date rites, genealogies, and temple/purāṇic narratives within a standardized cosmic framework.