Matsya Purana — Measures of Time: Caturyuga Computation
एषा चतुर्युगाख्या तु साधिका त्वेकसप्ततिः कृतत्रेतादियुक्ता सा मनोरन्तरमुच्यते //
eṣā caturyugākhyā tu sādhikā tvekasaptatiḥ kṛtatretādiyuktā sā manorantaramucyate //
This unit called a “cycle of four yugas”—seventy-one of them (with an additional portion), comprising Kṛta, Tretā, and the rest—is declared to be the interval of a Manu (a Manvantara).
It defines the time-structure governing creation’s recurring epochs: a Manvantara is measured as seventy-one four-yuga cycles (with an extra junctional portion), which frames the periodic renewals that culminate in larger dissolutions.
Indirectly, it situates dharma within cosmic time: royal and household duties are taught as yuga-conditioned, meaning conduct and governance are understood against the backdrop of changing ages within a Manu’s reign.
No direct Vāstu or ritual rule is stated; the takeaway is calendrical—ritual timing and Purāṇic reckoning often rely on yuga/Manvantara computations for situating sacred history and observances.