The Muhūrtas are traditionally calculated by assuming sunrise at 06:00 am on the Vernal Equinox, which is the Vedic New Year. Not all of the constellations cross the zenith, so that it is not in every case clear which constellation presides over the Muhūrta. Yet it is clear that one or more prominent features of the correlate constellations, from which the Muhūrtas draw their respective names, falls within the Celestial Longitude of the same, drawn from the Polar Axis.
Traditionally, it is common practice amongst Hindus to start or avoid starting significant tasks like religious ceremonies, etc. on the basis of the quality of a particular Muhūrta. The Vedic scriptures also generally recommend one or more Muhūrtas to perform rituals and practices. The most widely known example of this practice:
Brahma Muhūrta, approximately one and a half hours before sunrise or more precisely is 1Hr 36 Mins. i.e. 96 Minutes = 2 Muhūrta or 4 Ghaṭīkā, is recommended in all practices of yoga is traditionally considered most apt for meditation. However, it is clear from the associations of the names with specific constellations that the present Brahma-Muhūrta starts just before 6:00AM during the Vernal Equinox. At present, Jīva-Amṛta and Viṣṇu comprise the two twilight muhūrtas prior to sunrise.