Vishnupurana 2.8.61

2.8.61

rekhāprabhṛty athāditye trimuhūrtagate tu vai
prātaḥ smṛtas tataḥ kālo bhāgaś cāhnaḥ sa pañcamaḥ //

Fifteen twinklings of the eye (Nimeshas) make a Kásht́há; thirty Kásht́hás, a Kalá; thirty Kalás, a Muhúrtta (forty-eight minutes); and thirty Muhúrttas, a day and night: the portions of the day are longer or shorter, as has been explained; but the Sandhyá is always the same in increase or decrease, being only one Muhúrtta. From the period that a line may be drawn across the sun (or that half his orb is visible) to the expiration of three Muhúrttas (two hours and twenty-four minutes), that interval is called Prátar (morning), forming a fifth portion of the day. The next portion, or three Muhúrttas from morning, is termed Sangava (forenoon): the three next Muhúrttas constitute mid-day: the afternoon comprises the next three Muhúrttas: the three Muhúrttas following are considered as the evening: and the fifteen Muhúrttas of the day are thus classed in five portions of three each. But the day consists of fifteen Muhúrttas only at the equinoxes, increasing or diminishing in number in the northern and southern declinations of the sun, when the day encroaches on the night, or the night upon the day. The equinoxes occur in the seasons of spring and autumn, when the sun enters the signs of Aries and Libra. When the sun enters Capricorn (the winter solstice), his northern progress commences; and his southern when he enters Cancer (the summer solstice).