In the traditional Hindu day, time begins at sunrise — not midnight. This is the most important concept to understand why Panchang timings (Rahu Kalam, Yamagandam, Kuligai, Hora) differ from clock-based schedules. The Panchang day is the observed, natural day: from the moment the Sun rises above the horizon to the moment it sets.
The three daily inauspicious periods — Rahu Kalam, Yamagandam, and Kuligai Kalam — are all calculated as specific one-eighth fractions of the daytime. The daytime (sunrise to sunset) is divided into 8 equal octaves. Each weekday assigns a different octave to each inauspicious period. This means the timing is always relative to sunrise, never to an absolute clock time.
Sunrise varies significantly across India. India spans from 68°E (Dwarka, Gujarat) to 97°E (Arunachal Pradesh) — a difference of 29° of longitude, corresponding to nearly 2 hours of time difference. When the Sun rises in Itanagar (Arunachal Pradesh) at 4:30 AM, it does not rise in Dwarka until nearly 7:00 AM. All Panchang timings in these two cities will therefore differ by about 2.5 hours, even though they are in the same IST time zone.
Sunrise also varies seasonally at any fixed location. In Chennai, sunrise can range from about 5:42 AM (June) to 6:26 AM (December) — a variation of over 40 minutes. Combined with the change in day length (day duration ranges from about 670 minutes in winter to 790 minutes in summer), this means Rahu Kalam windows are not only offset but also longer or shorter depending on the season.
Hora timing works the same way. A Hora is a planetary hour — one-twelfth of the daytime, and one-twelfth of the night, each ruled by a planet in sequence. The first daytime Hora of any day starts exactly at sunrise and is ruled by the planet governing that weekday. All subsequent Horas follow in the Chaldean planetary order: Sun, Venus, Mercury, Moon, Saturn, Jupiter, Mars.