After Qingming: the Chen month begins

After Qingming the Month Pillar advances to Ren-Chen. This closes the solar-term category with a paired answer on each side of three different Jie: Lichun, Jingzhe, and Qingming. Each pair is a controlled experiment with exactly one variable changed.

Read together, the six cases make a single argument that a date-only chart cannot be trusted near the start of February, March, and April. Those are not edge cases invented for a test suite; they are the boundaries that produce most of the disagreement readers actually encounter between calculators.

The honest limit is that this suite covers three Jie in one year. It does not prove the engine is correct at every Jie across history, and it does not settle which convention a different school would use. It proves that this engine applies its stated rule consistently at the boundaries someone can check.

What that means in practice is that you should read these pages as evidence about behaviour rather than about truth. If your chart disagrees with this calculator near a solar term, these cases tell you exactly which rule produced the answer here, so you can decide whether you accept that rule. They cannot tell you which rule is correct, because that is a question about tradition rather than about arithmetic.