Matsya Purana — Measures of Time: Caturyuga Computation
एषा चतुर्युगावस्था मानुषेण प्रकीर्तिता चतुर्युगस्य संख्याता संध्या संध्यांशकैः सह //
eṣā caturyugāvasthā mānuṣeṇa prakīrtitā caturyugasya saṃkhyātā saṃdhyā saṃdhyāṃśakaiḥ saha //
Thus the condition (full measure) of the four-yuga cycle has been set forth by Manu in human terms; the total reckoning of a caturyuga is to be understood together with its twilight (sandhyā) and twilight-portions (sandhyāṃśa).
It frames cosmic time in precise units: a full caturyuga must include transitional twilight periods (sandhyā and sandhyāṃśa), which are crucial for understanding when major cosmic shifts—often linked with pralaya-like transitions—are said to occur.
By grounding dharma in a larger cosmic timetable, it implies that ethical and social duties are practiced within changing yuga-conditions; rulers and householders are expected to align conduct with the dharma appropriate to the age as defined by such yuga reckoning.
No direct vastu or iconographic rule is stated; the practical takeaway is calendrical—ritual timing and traditional computations should account for junction-periods (sandhyā/sandhyāṃśa) when calculating sacred cycles.