Matsya Purana — Measures of Time: Caturyuga Computation
एषा चतुर्युगाख्या तु साधिका ह्येकसप्ततिः क्रमेण परिवृत्ता सा मनोरन्तरमुच्यते //
eṣā caturyugākhyā tu sādhikā hyekasaptatiḥ krameṇa parivṛttā sā manorantaramucyate //
This unit called the cycle of four Yugas (caturyuga), when completed in due order seventy-one times (with the requisite additional interval), is termed a Manvantara—the period of a Manu.
It defines cosmic time-units: a Manvantara is measured as seventy-one cycles of four yugas (with an added interval), a framework used to situate creations and dissolutions within repeating epochs.
By fixing the scale of time in yugas and Manvantaras, it grounds dharma in an age-based context—royal and household duties are taught in the Purana as varying in emphasis across yugas, so knowing the epoch supports correct application of dharma.
No direct Vāstu or ritual procedure is stated; its significance is calendrical—ritual timing and Purāṇic prescriptions often depend on yuga/manvantara reckoning, so this verse supplies the underlying time-measure.