Why two calculators can disagree
BaZi calculators can give different charts for the same birth details because they make different choices about time. These five decide most differences:
- Whether birth time is corrected to true solar time
- Whether historical time zones and daylight saving are applied
- How the Month and Year Pillars follow solar terms
- When the Day Pillar changes during the late Zi hour
- Which gender and direction rule is used for Luck Pillars
How true solar time is calculated
Civil clocks use a standard time zone, while solar time follows the sun. The correction has three parts:
- Longitude offset: 4 minutes per degree between your birthplace and the time zone's standard meridian.
- Equation of time: a seasonal difference of roughly −14 to +16 minutes between apparent and average solar time.
- Daylight saving: when clocks were moved forward, that hour is removed.
For example, noon on January 1, 2000, in Beijing becomes about 11:42 solar time. Beijing is at 116.41°E while China Standard Time is set for 120°E, which accounts for about 14 minutes; the equation of time accounts for about 4 more.
How Daoly calculates a chart
| Decision | Daoly's convention |
|---|---|
| Time basis | Apparent solar time from birthplace longitude, time zone, daylight saving, and equation of time. You can use clock time instead when location is unknown. |
| Time zones | IANA time zone data, including historical offsets |
| Date rollover | If the correction crosses midnight, the corrected date is used |
| Year Pillar | Changes at Lichun (立春), not on January 1 or Lunar New Year |
| Month Pillar | Follows the 12 solar-term boundaries (节) |
| Late Zi hour (23:00–24:00) | Hour Pillar is the Zi hour of the next day's stem cycle; the Day Pillar changes at 00:00 |
| Luck Pillars | Direction from year-stem polarity and gender; start age measured from the nearest solar term |
The website calculator and the iPhone app use the same birthplace-aware correction and the same calendar algorithms (the JavaScript and Swift editions of the open-source Lunar library by 6tail).
Test case: the late Zi hour
Take June 15, 1990, using clock time. At 22:30 the chart is 庚午 壬午 辛亥 己亥. At 23:30 it is 庚午 壬午 辛亥 庚子: the Hour Pillar moves on, but the Day Pillar stays 辛亥. At 00:30 on June 16 the Day Pillar becomes 壬子.
Other schools change the Day Pillar at 23:00. Under that convention the Day Master at 23:30 would be 壬 rather than 辛. If you were born between 23:00 and 24:00, it is worth reading both.
Test case: a solar-term boundary
Lichun 2024 fell at about 16:27 Beijing time on February 4. At 16:00 that day, clock time gives 癸卯 乙丑 戊戌 庚申. At 17:00 it gives 甲辰 丙寅 戊戌 辛酉: both the Year and Month Pillars have changed. This is why a birth near a solar-term boundary deserves extra care.
Longitude correction by city
This table shows the longitude part of the correction under each city's current standard time. The equation of time, daylight saving, and historical time zones are added on top.
| City | Longitude correction (minutes) |
|---|---|
| Beijing | −14 |
| Shanghai | +6 |
| Ürümqi | −130 |
| Hong Kong | −23 |
| Taipei | +6 |
| Singapore | −65 |
| Seoul | −32 |
| Tokyo | +19 |
| New York | +4 |
| Los Angeles | +7 |
| London | −1 |
| Paris | −51 |
When it matters most
- You were born within about 30 minutes of a two-hour boundary
- You were born between 23:00 and 01:00
- You were born far from your time zone's standard meridian, such as western China
- You were born during daylight saving time
- You were born close to a solar-term change