Panchang in Chahārbāgh, Alborz Province, Iran Sat 22 Sep 2018

Shiva’s Pradosh puja is done in pradosha, the 3 muhurtas after sunset. Trayodashi runs from 01:41 on Sat 22 Sep to 03:43 on Sun 23 Sep, so only Saturday’s pradosha (18:04–20:26) falls within it. No text giving Pradosh Vrat’s time was found; pradosha is Tithika’s convention.

Pradosh Vrat: every date in Chahārbāgh

The Moon gains on the Sun about 12° a day. Each 12° of lead is one tithi: 144°–156° is the 13th, Shukla Trayodashi. A day takes the tithi in force at its sunrise, 05:54 here. Dwadashi ended at 01:41, before it. Trayodashi ends at 03:43 Sun, when the lead reaches 156°. The lead passed 0° at the new moon of 9 Sep, so the Moon is waxing: Shukla paksha.

The night of Fri 21 Sep ran from sunset 19:05 Fri to sunrise 05:54, 11 h 49 m. Cut into 15 muhurtas of 47 m, its 14th is Brahma Muhurta, the one before the last.

Daylight here, 05:54 to 18:04, splits into 8 choghadiyas of 1 h 31 m. Their names follow a fixed order that starts from the weekday, and each name is counted good, neutral or bad.

The night here, 18:04 to 05:55 Sun, splits into 8 choghadiyas of 1 h 28 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′. Dhanishtha spans 293°20′–306°40′; the Moon entered its pada 3 at 300°. It leaves at 17:30, for Shatabhisha.

Add the Sun's and the Moon's places among the stars: 154.9° + 300.9° = 455.8°, one full turn and 95.8°. Past the turn, cut into 27 parts of 13°20′, it gives the yoga: 93°20′–106°40′ is the 8th, Dhriti. It ends at 01:57 Sun, then Shula.

A karana is half a tithi: 6° of the Moon's lead, so the first half of Trayodashi is Kaulava and the second Taitila. Kaulava runs until 14:45, when the lead reaches 150°; Taitila until 03:43 Sun.

A Hindu day runs from one sunrise to the next, not midnight to midnight. This one runs 05:54 to 05:55 Sun, so the hours after midnight still belong to Shaniwara.

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: Simha gives Bhadrapada. The Sun enters Kanya on 17 Sep, inside the month. The next new moon, 9 Oct, finds it in Kanya and begins Ashwina.

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, began Bhadrapada. This Shukla paksha reads Bhadrapada in both counts.

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 182.1° round the stars. Vikram Samvat numbers that year 2075; Shaka Samvat numbers it 1940.