Amrit Kalam
A favourable stretch inside the day's nakshatra.
Each of the 27 nakshatras carries a conventional fraction of its own span that is treated as especially favourable. The window is computed from the actual start and end of the nakshatra in force, so it moves with the moon rather than with the clock, and it can fall at any hour — including the night — or not fall inside a given civil day at all. The fractions themselves are a received convention, not a computed quantity.