Sandhya belongs to a family of Sanskrit words built on the idea of joining, sandhi, the same term grammarians use for the junction of sounds between words. Applied to time it names every place where two periods meet, and the tradition used it at every scale at once. At the smallest, the day has its sandhyas, dawn, noon and dusk, the seams between night and morning, morning and afternoon, afternoon and night. These are the hours of the sandhya worship, the daily observance of reciting the Gayatri at the junctions, on the old intuition that seams in time are open: what is neither day nor night belongs fully to neither order and is therefore potent, apt for prayer, and in folklore apt also for danger.
Mythology exploits the same logic; the man-lion Narasimha kills the demon at twilight precisely because the boon protecting him named day and night but not their joint. At the largest scale the same word names the dawns and twilights of the yugas and the junctions between manvantaras, the great seams at which one order of the world gives way to another. The unification is not an accident of vocabulary but a piece of metaphysics: the cosmos and the day are the same shape at different magnifications, and the practitioner standing at dusk to recite is occupying, in miniature, the very kind of threshold through which whole ages pass.