Bazi Calculation Methodology

Adopted Calculation Conventions

ChiBuddy follows a Four Pillars convention that uses Li Chun for the year boundary and Jie solar terms for month boundaries. This guide explains the chart-changing conventions visible to users; it does not publish implementation details.

1. Year Pillar: Li Chun Boundary

The year pillar changes at Li Chun (立春, the Beginning of Spring solar term), which falls on approximately February 3–5 in the Gregorian calendar. This means:

• •A birth before Li Chun uses the previous year's ganzhi for the year pillar, even if the Lunar New Year has already passed.
• •A birth after Li Chun uses the new year's ganzhi, even if the Lunar New Year hasn't arrived yet.
• •The Chinese Zodiac calculator shows both the folklore zodiac (Lunar New Year based) and the astrology zodiac (Li Chun based) for comparison.

2. Month Pillar: Solar Term Boundaries

The month pillar is determined by solar terms (节气), not by lunar months. Each Bazi month begins at a specific solar term (节气), typically the "Jie" (节) terms:

• •寅月: 立春 (Beginning of Spring)
• •卯月: 惊蛰 (Awakening of Insects)
• •辰月: 清明 (Clear and Bright)
• •巳月: 立夏 (Beginning of Summer)
• •午月: 芒种 (Grain in Ear)
• •未月: 小暑 (Minor Heat)
• •申月: 立秋 (Beginning of Autumn)
• •酉月: 白露 (White Dew)
• •戌月: 寒露 (Cold Dew)
• •亥月: 立冬 (Beginning of Winter)
• •子月: 大雪 (Major Snow)
• •丑月: 小寒 (Minor Cold)

The month pillar changes at the solar-term transition time, not simply at the start of a calendar date or lunar month.

3. Zi-Hour and Day Boundary

The traditional Chinese 12-shichen (时辰) system divides the day into 12 two-hour periods. The Zi-hour (子时) spans 23:00–00:59. ChiBuddy uses this mapping:

Note: The time ranges above refer to local apparent solar time after historical timezone, DST, longitude, and Equation of Time handling under ChiBuddy’s policy.

For the day pillar, ChiBuddy uses the midnight (00:00) boundary convention: the day boundary is midnight (00:00) — not 23:00. This means a birth at 23:30 on January 15 still uses the January 15 day pillar. The entire Zi-hour (23:00–00:59) is assigned to the same day pillar as the calendar date of birth.

4. True Solar Time: Longitude + Equation of Time

When birthplace data is available, ChiBuddy determines True Solar Time before calculating the chart. The correction combines the birthplace longitude, the historical timezone and daylight saving time that applied locally, and the Equation of Time.

The correction is applied only when the required birthplace and timezone information can be determined. Otherwise, the chart keeps the entered local birth time and indicates that no correction was used.

5. Timezone and Daylight Saving Time

ChiBuddy uses the IANA Time Zone Database to identify the historical timezone rules that applied at the birthplace and birth date.

Key behaviors:

• •Historical daylight saving time (DST) rules are automatically applied. For example, a birth in New York in July 1944 would use UTC−4 (EDT was in effect), while a birth in January would use UTC−5 (EST).
• •Standard time and daylight saving time are distinguished before True Solar Time is determined.
• •Non-integer timezone offsets (e.g., UTC+5:30 for India, UTC+5:45 for Nepal) are handled correctly.
• •Users should enter the birth time as it was recorded locally at the birthplace. No manual DST adjustment is needed.

6. 10-Year Life Cycle (大运, traditionally Dayun) Start Age and Direction

ChiBuddy determines the 10-Year Life Cycle start age and direction using the following traditional conventions:

Start Age

Start age is measured from the birth time to a Jie solar-term boundary: forward direction uses the next Jie and backward direction uses the previous Jie. The interval follows the traditional conversion of three days to one year.

The page displays the nominal traditional age (虚岁), so it may differ from tools that show a precise year-month-day offset.

Direction (Forward / Backward)

The direction is determined by the yin/yang of the year stem and the gender:

Each 10-Year Life Cycle spans 10 years. ChiBuddy calculates 8–9 10-Year Life Cycles, covering approximately 80–90 years of life.

7. Why Different Calculators Give Different Results

If you compare ChiBuddy's results with other Bazi calculators, you may see differences. Here are the most common causes:

1. 1.True Solar Time vs. Standard Time: Some calculators do not apply longitude or EoT corrections. A birth at 12:00 in a city far from the timezone meridian could shift by 10+ minutes, potentially changing the hour pillar.
2. 2.Daylight Saving Time: If a calculator does not use the IANA timezone database, it may mishandle historical DST, causing a 1-hour shift.
3. 3.Zi-hour Day Boundary: The 23:00 vs. midnight convention can shift the day pillar by one position for births between 23:00 and 00:59.
4. 4.Li Chun vs. Lunar New Year: Using the wrong year boundary affects the year pillar for births in late January to mid-February.
5. 5.Solar-term boundary convention: Different tools may adopt different solar-term data or boundary conventions, which can affect the month pillar for births near a transition.

FAQ

References & Sources

• •Hong Kong Observatory — weather.gov.hk — Public background on the 24 solar terms.
• •IANA Time Zone Database — iana.org/time-zones — The authoritative source for timezone and DST rules worldwide.
• •NASA OMI reference material — nasa.gov — Astronomical background related to the Equation of Time.

Changelog