A BaZi chart is only as good as the calendar arithmetic behind it. This page walks through what Fate Hall does with your birth details, step by step, and how each step is checked before every release. The full list of data sources and licences is on the Sources & methods screen.
1. From your clock to Universal Time
You enter the date and time on your birth certificate and your birthplace. Fate Hall first turns that local clock time into Universal Time, using the time zone of your birthplace on that exact date from your browser's time zone database. That removes daylight saving time and catches historical changes — Korea's use of UTC+8:30 in parts of the 20th century, for example, or summer time in the 1950s and 1980s. Nothing is sent to a server for this; your browser already carries the data.
2. Local solar time
Next comes the correction from clock time to solar time. By default Fate Hall applies the longitude correction (local mean time); you can also choose true solar time, which adds the equation of time, or leave the clock time as it is. The reasons and the size of the corrections are explained in True Solar Time. The BaZi screen always shows your chart under every choice, so you can see whether it matters for you.
3. The solar terms
The year and month pillars change not on calendar dates but at solar terms — the moments when the Sun reaches every 15° of longitude along its path. Fate Hall calculates these moments astronomically. Its table of solar terms for 1900–2050 has been checked against the figures published by the Korea Astronomy and Space Science Institute (KASI): all 3,624 terms agree to the minute.
4. The four pillars
- Year pillar — counted from the Beginning of Spring, so January and early-February births belong to the previous year.
- Month pillar — the branch is fixed by which solar month you were born in, and the stem follows from the year stem.
- Day pillar — counted on the unbroken sixty-day cycle from the Julian day number of your birth date, after the time correction, applying your chosen Rat-hour rule near midnight.
- Hour pillar — the branch is the two-hour period of your corrected birth time, and the stem follows from the day stem.
5. Luck Pillars, Ten Gods and the rest
From the four pillars, Fate Hall derives the direction and starting age of your Luck Pillars from the exact solar-term moments, the Ten Gods, the life stages, symbolic stars and the element balance. Every table on the guide pages is generated from the same code, and an automated test compares them with the app before each release.
6. The lunar calendar
Readings that need a lunar date — Tojeong Bigyeol, Zi Wei Dou Shu and lunar birthdays — use the Korean lunar calendar for 1900–2050 published by KASI, from an open-source data set that has been compared with KASI's own service. The Korean and Chinese lunar calendars agree on almost every day, but because they are calculated for different time zones they differ on a little over 2,000 of the roughly 55,000 days in that range. Fate Hall follows the Korean calendar and says so on the results.
7. Western astrology
Planetary positions come from NASA JPL's approximate orbital elements (accurate to about one degree for the planets) and the Moon from Jean Meeus's abbreviated formulas (about 0.3°). The Ascendant and Midheaven are calculated from sidereal time for your birthplace. These are checked against published astronomical events, such as conjunctions and oppositions of Venus, Mars, Mercury, Jupiter and Saturn, to within a few hundredths of a degree at the moment of the event.
How the checks work
Before each release, an automated test suite runs thousands of comparisons. The most important:
- Four pillars against an independent implementation. Several thousand random births, each under several combinations of time basis and Rat-hour rule, are calculated by Fate Hall and by a separate open-source BaZi library written by other people. From April 1908 onward there are no differences. Before 1908 Korea had no standard time, and the two programs assume different things about clock time, so some hour pillars differ for births before then.
- Luck Pillar starting ages, Ten Gods, Void branches and lunar dates are compared with the same library, with no differences.
- Solar-term boundaries are tested for births a couple of minutes before and after each term in several time zones, to make sure the year and month pillars switch at the right moment.
- Screens and pages are rendered in a real browser at many screen sizes and in light and dark mode, and checked for errors and overflow.
Honest limits
- Fate Hall's calendars cover births from 1900 to 2050.
- Traditional schools disagree on several points — the Rat-hour rule, time correction, some symbolic-star tables. Where they do, Fate Hall shows the options rather than claiming one is correct.
- All of this makes the chart accurate. It does not make a reading true. Fate Hall is for entertainment and reflection.
Related guides
- True Solar TimeWhy BaZi uses the Sun's position instead of clock time: longitude correction, the equation of time and daylight saving, with tables for major cities and every month.
- Luck PillarsHow the ten-year Luck Pillars are built from your month pillar, why they run forward or backward, how the starting age is calculated from the solar terms, and how to read them.
- The Sixty Jiazi and NayinAll sixty stem-branch pairs from Jia Zi to Gui Hai, with their animal, Nayin element and years from 1924 to 2043, and how the cycle counts years, months, days and hours.
- The Rat Hour RuleWhy births between 23:00 and midnight can have two different day pillars, how the early and late Rat-hour rules work, and the full table of hour pillars.
- Zi Wei Dou Shu: Palaces and StarsAn introduction to Zi Wei Dou Shu (Purple Star Astrology): the twelve palaces of life, the fourteen main stars, the five element bureaus and how the chart is placed from your lunar birthday.