Same birth.
Different chart?

Why BaZi calculators can disagree

A chart is not produced from a birth date alone. Software must first turn a local date and time into one precise moment, then decide where that moment falls against calendrical boundaries. If two calculators make different choices at either step, one or more pillars can change.

Timezone and daylight saving: a city can have different UTC offsets in different years. A fixed “UTC−5” guess is not equivalent to the historical rules for America/New_York. Solar terms: Year and Month Pillars can change at Li Chun or a Jie boundary rather than at a civil calendar boundary. Day boundary: some schools move the BaZi day at 23:00; others use midnight. Solar correction: apparent solar time depends on longitude and the equation of time, while civil time is shared across a timezone.

A five-step comparison checklist

1. Use the same Gregorian birth date, recorded local time, and birthplace in both tools.
2. Confirm the historical timezone and whether daylight saving was active on that date.
3. Check whether each calculator applies local civil time or a longitude-based solar correction.
4. Compare the declared Year, Month, Day, and Hour boundary rules.
5. Find the first pillar that differs, then test whether the birth moment is close to that pillar's boundary.

The rules used in version 1

Year pillar: the year changes at Li Chun (立春, Start of Spring), not January 1 and not Lunar New Year. Month pillar: months change at the twelve Jie solar-term boundaries. Day pillar: the day changes at local civil midnight. A birth at 23:00–23:59 stays on that civil date. Hour pillar: calculated in two-hour Earthly Branch periods when an exact birth time is provided.

Why we do not pretend civil time is true solar time

A true-solar calculation requires an accurate longitude and a declared equation-of-time convention. Asking only for a city while silently snapping to a centroid creates false precision. We prefer an explicit limitation over a more impressive but ambiguous control. A future version may add true solar time only with testable location and correction rules.

What “Five Elements balance” means here

The visual balance is a simple count of the heavenly stem and earthly branch in each visible pillar. A complete four-pillar chart therefore totals eight characters. Hidden stems are shown separately and are not folded into a secret weighted score. This is not a claim about chart strength, favorable elements (yong shen), or destiny quality.

How correctness is checked

The calculation engine is server-side and pinned to a reviewed library version. We keep fixed regression vectors for known dates, a specific test for the 23:00 boundary, tests for invalid dates, IANA timezones, and nonexistent daylight-saving times, and a test ensuring an unknown birth time returns only three pillars. Every method change must increment the policy version and update those vectors.

Published reference vector

For 23 December 2005 at 08:37 local civil time in Asia/Shanghai, policy v1 returns Year 乙酉, Month 戊子, Day 辛巳, and Hour 壬辰. This vector is also asserted in the backend test suite.

Interpretation boundary

The reflection card is deterministic copy selected from the Day Master's element, polarity, and the focus you choose. It is designed to explain its source. It does not predict a medical outcome, investment return, legal result, relationship fate, or unavoidable future event.

Data and implementation boundary

Birth date, time, timezone, chart, and interpretation are processed in server memory and are not written to the analytics table. We store only allow-listed funnel event names, page path, a random browser session identifier, user agent, timestamp, and a one-way IP hash used for abuse control. The browser receives calculated output, not lookup tables, weights, prompts, or the calculation library.

Frequently asked questions

Why do two BaZi calculators give different results?

They may use different timezone histories, daylight-saving handling, solar-term moments, day-change conventions, or true-solar-time corrections. Compare the declared method before deciding that either chart is wrong.

Can daylight saving time change a BaZi chart?

Yes. Historical daylight-saving rules change the UTC offset of a recorded local time and can move the Hour Pillar near a two-hour boundary. Pillar Atlas resolves the birth moment with the selected IANA timezone.

Does Pillar Atlas use true solar time?

No. Policy v1 uses local civil time in the selected IANA timezone and does not apply a longitude-based true-solar correction. The limitation is published so charts can be compared reproducibly.

What happens if my birth time is unknown?

Pillar Atlas returns the Year, Month, and Day Pillars and does not invent an Hour Pillar. The Day Master remains available, while hour-dependent claims should remain open.

Timekeeping references

The IANA Time Zone Database records changes to civil-time offsets and daylight-saving rules. The U.S. Naval Observatory's equation-of-time guide explains why apparent solar time varies by longitude and date. These sources explain the timekeeping layer; Pillar Atlas's BaZi boundary choices remain the versioned policy published above.