Matsya Purana — Characteristics of Dvāpara and Kali Yugas
युगस्वभावाः संध्यासु अवतिष्ठन्ति पादतः संध्यास्वभावाः स्वांशेषु पादेनैवावतस्थिरे //
yugasvabhāvāḥ saṃdhyāsu avatiṣṭhanti pādataḥ saṃdhyāsvabhāvāḥ svāṃśeṣu pādenaivāvatasthire //
The inherent qualities of each Yuga persist in the twilight junctions (sandhyās) only to the extent of a quarter; and the qualities of the sandhyās themselves likewise remain established within their own portions by just a quarter.
It describes how cosmic time transitions work: during yuga-junctions (sandhyās), the full traits of a yuga do not manifest completely, but only partially (by a quarter), indicating graded change rather than abrupt shifts—an idea often paired with broader Purāṇic cyclic creation and dissolution.
Because dharma conditions shift gradually at yuga boundaries, rulers and householders must adapt governance, discipline, and ritual emphasis to transitional moral climates—recognizing that mixed qualities prevail in sandhyā periods rather than a single yuga’s purity.
While not directly about Vāstu or iconography, it supports ritual timing doctrine: sandhyā is a liminal time where practices are often prescribed with special care, because qualities are mixed and only partially established—reinforcing the importance of correct sandhyā-related observances.