After Jingzhe: the Mao month begins

Two days after the previous case, the Month Pillar has moved from Geng-Yin to Xin-Mao while the Year Pillar stays put. That asymmetry is the point: the year and the month change at different solar terms, so crossing one boundary does not imply crossing the other.

For a reader, the consequence is that a chart can be partly stale. If you were born on 6 March 2026, the year pillar is the same as someone born in February, but your month pillar is not. Comparing charts with a friend born a week earlier will show one shared pillar and one that differs, which is exactly what the rules predict.

This case exists so the boundary has a pinned answer on both sides. It is the reference we check against when the engine or its timezone data is upgraded, which matters because a silent shift in solar-term timing would move the Month Pillar for everyone born near a Jie.

That upgrade risk is not hypothetical. Solar-term timestamps come from ephemeris data, and different sources can differ by a minute or two. For a record sitting hours from a Jie this is irrelevant. For a record sitting a few minutes from one, a library update is enough to change the Month Pillar. Pinning these cases is how that class of change gets caught at build time rather than by a reader noticing a different chart.