- Confirm the same local date, clock time and birth city were entered.
- Compare historical UTC offset, DST and true-solar-time policy.
- Then compare solar-term, year/month boundary, Zi hour and lunar-conversion rules.
1. Same person, different input
A place name without a city, a rounded hour, an unmarked leap month or a silently applied daylight-saving offset can produce a different chart before any metaphysics rule is involved.
2. Time-standard differences
| Check | What can change | Evidence to request |
|---|---|---|
| Historical time zone | Date, day or hour | UTC offset for city and date |
| Daylight saving | Hour, sometimes date | Applied offset in minutes |
| True solar time | Hour, sometimes date | Longitude and equation of time |
3. Calendar-boundary differences
BaZi months follow solar terms, not Gregorian month starts. The year commonly changes at Li Chun under the selected convention. Near an exact transition, a date-only calculator cannot honestly choose one side without a time.
4. Zi hour and day rollover
Schools differ over whether the day changes at 23:00, midnight or under another Zi-hour convention. A birth between 23:00 and 01:00 therefore needs the rule named explicitly.
5. Same chart, different interpretation
Even when all four pillars match, systems may weight strength, structure and Regulating Element differently. That is an interpretation disagreement, not a calculation disagreement. Keep the two questions separate.