These guides are written for anyone: someone checking today’s Rahu Kaal for the first time, a student of astronomy, or a neighbour curious about how another community’s calendar works. Each guide explains the idea in plain words, shows how it’s calculated, and points out where traditions differ.
You’ll find guides on the Hindu panchang and muhurat windows, the Islamic calendar and prayer times, Jain tithis and Navkarshi, the Parsi Roj and Mah, the Nanakshahi calendar, and everyday tools. For an overview of each calendar system, see calendars. For what OIOA calculates, see features.
- What is a tithi?A tithi is the time the Moon takes to gain 12° on the Sun. Learn how tithis are counted, why they vary in length, and what kshaya and vriddhi tithis are.
- Shukla and Krishna pakshaWhat Shukla and Krishna paksha mean, how they divide the lunar month, why their order differs in Amanta and Purnimanta calendars, and key observances.
- The 27 nakshatrasThe 27 nakshatras explained: what they are, their names, how padas link them to the zodiac, why calendars differ on timing, and how they're used today.
- Purnimanta vs AmantaWhy Hindu lunar months end at full moon in some regions and at new moon in others, which regions use which, and how it changes festival month names.
- Adhik maas explainedAdhik maas (Purushottam or Mal maas) is the extra lunar month that keeps the Hindu calendar with the seasons. How it's worked out and what it means.
- Sankranti and solar monthsWhat a sankranti is, how solar months work in Tamil, Malayalam, Bengali, Odia, Assamese and Punjabi calendars, and why Makar Sankranti is in mid-January.
- Udaya tithi and the sunrise ruleThe udaya tithi rule: why the tithi at sunrise names the whole day, how kshaya and vriddhi tithis arise, and why some festivals use other rules.
- Vikram, Shaka and Gujarati New YearWhy India has many new years: Vikram Samvat in Chaitra and after Diwali, Shaka Samvat at Ugadi and Gudi Padwa, and regional new years.
- What is Rahu Kaal?Rahu Kaal explained: what it is, the origin of Rahu, how it's calculated from sunrise and sunset for each weekday, and how Yamaganda and Gulika relate.
- How Choghadiya worksA step-by-step guide to Choghadiya: the seven types, the day and night sequences, the first Choghadiya for each weekday, and a worked example.
- What is Hora?Hora explained: the planetary hours of Indian tradition, the Chaldean order behind them, how they set the order of the weekdays, and how to work them out.
- Abhijit muhurat explainedAbhijit muhurat explained: the 8th of 15 daytime muhurtas, centred on local noon. How to calculate it, why it's not 12:00, and the Wednesday exception.
- What is Panchak?Panchak explained: the Moon's passage through the last five nakshatras, how long it lasts, types such as Raj and Mrityu Panchak, and customs.
- Why sunrise times differOne clock, many sunrises: how longitude, latitude, season and the equation of time change local timings, and why it matters for calendars.
- Hijri vs GregorianHow the Hijri calendar differs from the Gregorian: lunar months, a 354-day year, why Ramadan moves 11 days earlier each year, and converting years.
- Moon sighting and Umm al-QuraWhy Islamic months can start on different days: local and global sighting, astronomical calculation and the Umm al-Qura calendar, explained neutrally.
- Shia and Bohra calendarsHow Shia and Bohra (Dawoodi, Misri) communities reckon the Islamic calendar, the Fatimid tabular system, and why dates can differ from sighting.
- How prayer times are calculatedHow Fajr, Dhuhr, Asr, Maghrib and Isha times are calculated from the Sun's position, why methods like Karachi and MWL differ, and high-latitude rules.
- Asr: Hanafi and standardWhy there are two Asr times, the standard and Hanafi shadow rules, how each is calculated, and how far apart they are in India.
- Jumma: the Friday prayerWhat Jumma (Jumu'ah) is, when its time begins, how khutbah and congregation times are set by mosques, and how to plan a Friday around it.
- How Qibla is calculatedHow the Qibla is worked out as a great-circle bearing to the Kaaba, the formula in plain words, example bearings for Indian cities, and compass tips.
- Jain tithi and the sunrise ruleHow Jain calendars decide a day's tithi, the parva tithis kept for fasting, and how traditions handle kshaya and vriddhi tithis differently.
- 28 Jain nakshatras and AbhijitWhy the Jain tradition counts 28 nakshatras, where Abhijit fits, how the spans differ from the 27-nakshatra system, and how the list is used today.
- Navkarshi explainedWhat Navkarshi is, why it's 48 minutes after sunrise, how to work out Navkarshi time for your city, and related pachchakhan such as Porsi and Chauvihar.
- Jain sects and their calendarsShwetamber and Digamber traditions, gachhs such as Khartargachh, Sthanakwasi and more, and why their calendars can place observances differently.
- Parsi Roj and MahThe 30 Roj (day) names and 12 Mah (month) names of the Parsi calendar, the five Gatha days, and the jashans when a day and month share a name.
- Shahenshahi, Kadmi and FasliThe history of the three Parsi calendars: why Shahenshahi and Kadmi are a month apart, how the Fasli calendar keeps Navroz on 21 March, and how to choose.
- Gahambars explainedThe six Gahambars: their names and seasons, the creation each honours, the jashan and communal meal, and why dates differ between calendars.
- Nanakshahi calendar historyThe Nanakshahi calendar's history: the Bikrami tradition, Pal Singh Purewal's design, adoption in 2003, the 2010 amendment, and why both are followed.
- Planning long weekendsHow India's holiday lists work, gazetted, restricted, state and bank, how to spot long weekends and bridge days, and tips for planning leave.
- GST calculation basicsHow to calculate GST: adding GST to a price, extracting GST from an inclusive price, CGST, SGST and IGST, the current slab structure, and common mistakes.
- Traditional horoscope matchingHow traditional kundli matching works, Ashtakoot guna milan and South Indian porutham, what it is and isn't, and how to use it with wisdom.
- How offline calendar apps workHow an app shows panchang, prayer times, Qibla and calendar dates without the internet: the astronomy, the inputs, accuracy and privacy.