Answer first
A BaZi chart does not start with a self-defining value called “birth time.” It starts with a local clock reading, a place, a time-zone history, and a set of disclosed calculation rules. The IETF’s time-zone model also warns that a local clock time can map to more than one instant when UTC-offset or daylight-saving changes overlap. Different choices at any of those layers can change the Four Pillars before interpretation begins.
Independence and self-links. Sky Atlas is independently published and edited by Zhou Zhe. It also operates the portrait tool linked at the end of this guide, so that is a self-link rather than an independent endorsement. This guide has no sponsorship, paid placement, or affiliate relationship with any cited third-party calculator, code library, or source. Sky Atlas has no affiliation with any temple, lineage, professional association, government body, certification organization, or regulator. It does not provide personalized readings, claim that any school is official, or decide which interpretation is uniquely correct.
The same typed local time can produce different Four Pillars results when calculators resolve the time zone differently, use civil time or apparent solar time, choose a different year or late-Zi day boundary, calculate solar-term boundaries differently, or apply a different implementation rule. Those choices can change the structure before interpretation; school or editorial readings can then differ even when the pillars match.
Before comparing charts, compare their assumptions. The calendar and time-zone guide explains the date layer, while the Four Pillars guide explains the structural terms. A result is reproducible only when both the normalized input and convention are stated.
Six layers that can change a result
The first row constrains the timestamp; the next four rows choose how the calendar coordinate is calculated; only the final row interprets the resulting structure.
Public implementations expose different conventions
These are public implementation choices, not evidence that one lineage is universally correct. They show that the same named convention does not imply one universal implementation rule, so a calculator must disclose its own rules.
Within Sky Atlas, an element count is treated as a display of the calculated structure, not as a prescription that something is “missing” and must be corrected. The portrait reading guide explains how the project keeps calculation, tradition, and editorial interpretation separate.
Diagnostic map: find the layer that changed
Three visible examples
These examples are project-engine outputs generated by the Sky Atlas methodology. They demonstrate sensitivity to one stated convention at a time; they do not prove that one tradition or interpretation is universally correct.
Lichun changes the year and month pillars
The year pillar remains 癸卯.
The year pillar becomes 甲辰.
23:30 can belong to different day positions
Day pillar 戊戌.
Day pillar 丁酉.
Daylight saving and longitude can change the hour pillar
Hour pillar 壬午.
Hour pillar 辛巳.
How to compare two calculators fairly
- 01 Use the same city and IANA zone.
A nearby city name is not enough when historical offsets or daylight saving differ.
- 02 Use the same clock time or the same solar-time correction.
“True solar time” can hide different longitude and daylight-saving assumptions.
- 03 Check the stated year and day boundaries.
Ask whether the year changes at Lichun and the day at 23:00 or 00:00.
- 04 Compare the four pillar strings before the interpretation.
If the structural output already differs, the interpretive disagreement has a prior cause.
- 05 Ask which text or school supports the reading.
A structural calculation does not select one interpretation by itself.
- 06 Record the implementation and version.
Two programs can implement the same named convention differently or change it across releases.
A reproducibility test
A reproducible calculator should expose the local date and time, IANA time zone, UTC offset, time basis, year boundary, late-Zi rule, solar-term treatment, implementation version, and interpretation source. Without those fields, a different result cannot be diagnosed.
What this does not mean
- A different result does not prove that one calculator is fraudulent.
- Showing a convention does not prove that its interpretation is scientifically valid.
- For a time near a stated year, month, day, or solar-time boundary, more decimal places do not resolve which convention was used.
- A variant is not a second destiny hidden behind the first.
- No calculator can establish medical, legal, financial, or safety outcomes.
The standard is reproducibility: could another reader reconstruct the same output from the same input and stated rules? That is a narrower claim than saying a chart determines a life outcome, and a much more useful one.
Sources
- IANA, Time Zone Database, accessed 2026-10-21.
- IETF, RFC 9557: Date and Time on the Internet, accessed 2026-10-21.
- Hong Kong Observatory, Gregorian-Lunar Calendar Conversion Table and The 24 Solar Terms, accessed 2026-10-21.
- NOAA Global Monitoring Laboratory, Apparent Solar Time, accessed 2026-10-21.
- 6tail, lunar-python source code, accessed 2026-10-21.
- overtrue, chinese-calendar conventions, accessed 2026-10-21.
Source review: 2026-10-21. See corrections or contact Sky Atlas with a source or reproducibility issue.
Next step
Inspect the convention behind one result.
Inspect the convention behind one result in the portrait tool. Then compare the receipt with the related calendar and time-zone guide and Four Pillars guide.
Compare conventions in the portrait