Synthetic example: the exact term instant, not the printed date, decides the side.
Li Chun is a solar term, not a date label
Li Chun, commonly translated as Beginning of Spring, is one of the twenty-four solar terms. It occurs when the Sun reaches the corresponding ecliptic longitude. Calendars often print it on February 3, 4, or 5, but the calculation boundary is a precise instant rather than the whole local day.
MingWeave compares the resolved birth instant with that instant. A birth before Li Chun remains in the preceding BaZi year; a birth at or after the boundary enters the new year. This rule applies even though the Gregorian year already changed on January 1 and Lunar New Year may occur on another date.
Why three familiar ‘new years’ can diverge
January 1 is a civil calendar boundary. Lunar New Year begins a lunisolar calendar year. Li Chun is a solar-term boundary used by MingWeave’s Four Pillars method. They belong to different systems, so treating them as interchangeable can move the Year Pillar for births in the intervening weeks.
The difference is not resolved by choosing the most familiar holiday. A calculator needs a documented convention. Some traditions use other rules, but a reproducible product should state which one it implements, retain the precise term timestamp, and warn when an input lies close enough that record accuracy matters.
A boundary example
Imagine Li Chun occurs at 16:20 in the resolved working time. A synthetic birth at 16:19 belongs to the previous BaZi year under this rule; one at 16:20 belongs to the new year. Entering only the date would be insufficient because both births share the same calendar label but fall on opposite sides.
If True Solar Time is selected, MingWeave still compares the physical UTC birth instant with Li Chun; solar correction does not move the Year boundary. The trace preserves civil and apparent-solar clocks for Day and Hour rules while keeping astronomical boundary comparison on the real instant.
- Compare instants, not printed dates.
- Use the historical local time-zone conversion.
- Treat near-boundary records as sensitive to clock precision.
How to audit a Year Pillar disagreement
Ask which boundary the other calculator uses, what Li Chun timestamp and ephemeris it uses, how it converted the birth location, and whether it applied solar time. Compare the birth instant and boundary in one time scale. A bare Year Pillar is not enough evidence to identify the cause.
MingWeave shows the previous and next solar terms, distance to the relevant boundary, Method Profile, engine version, and solar-term data version. If the record is close to Li Chun, preserve the warning when sharing the result. The chart should communicate the condition, not erase it.