Panchang in Chahārbāgh, Alborz Province, Iran Sun 2 Sep 2018

Krishna’s birth is celebrated at nishitha, the midnight muhurta. Ashtami runs from 19:47 on Sun 2 Sep to 18:20 on Mon 3 Sep, so only Sunday night’s nishitha (00:44 Mon–01:28 Mon) falls within it.

Janmashtami: every date in Chahārbāgh

The Moon gains on the Sun about 12° a day. Each 12° of lead is one tithi: 252°–264° is the 22nd, Krishna Saptami. A day takes the tithi in force at its sunrise, 06:39 here. Shashthi ended at 20:44 Sat, before it. Saptami ends at 19:47, when the lead reaches 264°. The lead passed 180° at the full moon of 26 Aug, so the Moon is waning: Krishna paksha.

The night of Sat 1 Sep ran from sunset 19:34 Sat to sunrise 06:39, 11 h 5 m. Cut into 15 muhurtas of 44 m, its 14th is Brahma Muhurta, the one before the last.

Daylight here, 06:39 to 19:33, splits into 8 choghadiyas of 1 h 36 m. Their names follow a fixed order that starts from the weekday, and each name is counted good, neutral or bad.

The night here, 19:33 to 06:39 Mon, splits into 8 choghadiyas of 1 h 23 m. Their names follow a fixed order that starts from the weekday, and each name is counted good, neutral or bad.

The fixed stars divide the Moon's path into 27 nakshatras of 13°20′, each in 4 padas of 3°20′. Krittika spans 26°40′–40°00′; the Moon entered its pada 2 at 30°. It leaves at 19:48, for Rohini.

Add the Sun's and the Moon's places among the stars: 135.5° + 32.6° = 168.1°. The sum, cut into 27 parts of 13°20′, gives the yoga: 160°00′–173°20′ is the 13th, Vyaghata. It ends at 15:25, then Harshana.

A karana is half a tithi: 6° of the Moon's lead, so the first half of Saptami is Vishti and the second Bava. Vishti runs until 08:19, when the lead reaches 258°; Bava until 19:47.

A Hindu day runs from one sunrise to the next, not midnight to midnight. This one runs 06:39 to 06:39 Mon, so the hours after midnight still belong to Raviwara.

An amanta month runs from new moon to new moon and takes its name from the rashi the Sun is in at the new moon that begins it: Karka gives Shravana. The Sun enters Simha on 17 Aug, inside the month. The next new moon, 9 Sep, finds it in Simha and begins Bhadrapada.

A purnimanta month runs from full moon to full moon, so its waning half comes first. It shares its name with the amanta month that its second half falls in: the new moon of 9 Sep, with the Sun in Simha, begins Bhadrapada. Until then this Krishna paksha reads Bhadrapada here, and Shravana in the amanta count.

A samvat year begins with Chaitra, the month whose new moon finds the Sun in Meena: 17 Mar 2018, 16:41 Sat. Since then the Sun has moved 162.7° round the stars. Vikram Samvat numbers that year 2075; Shaka Samvat numbers it 1940.