How the schools differ, and which choices the engine makes

Anyone who has had a chart calculated twice knows that two practitioners can disagree on the hour pillar, on the hidden stems of a branch or on whether the Day Master is strong. This is not carelessness. The classical texts were written over a thousand years in different regions, and at several points they read the same rule differently. A calculation engine has to choose at every such point, and the honest thing is to say which choice it made.

This page lists the forks that matter for a reader; the full list is on the calculation method page, and every chart in the app records the engine version it was calculated with.

Time and calendar

• Day boundary. Does the day change at midnight or at 23:00, when the 子 double hour begins? The engine applies the 23:00 day-boundary rule: the whole 子 hour (23:00–01:00) belongs to the next day. The alternative in the settings is the split-子 rule (分早晚子時): the day pillar changes at midnight, while the hour pillar after 23:00 already takes the next day's stem.
• True solar time. On by default for the day and hour pillars; the alternative is zone time (why it matters).
• Solar terms. Computed astronomically and compared in absolute time, not by calendar date.

BaZi

• Hidden stems. The engine follows the table of month-commanding stems (人元司令) with residual stems: 子 holds 癸 and 壬, 卯 holds 乙 and 甲, and so on; the full table is on the method page. The shorter table of 淵海子平, where 子 holds only 癸 and 卯 only 乙, has been measured on a set of charts: it changes the verdict in three charts out of a hundred and twenty, and the switch is planned with a new engine version.
• Day Master strength. The classical three criteria decide; a weighted share of supporting elements overrides them only at the extremes, and the app says when the two disagree (how it is judged).
• Luck pillars. Forward for a yang year and a man or a yin year and a woman, backward otherwise; the start age from the distance to the solar term, three days per year (how they are built).

Qi Men Dun Jia

• Layout method. The chart is built by the split-and-supplement (拆補) method by default: the solar term gives three candidate layouts, and the day's position in the ten-day cycle picks one of them. The alternative is available in the settings.
• The centre palace. When the fifth palace has to be read, the engine follows the school that sends it to the second palace; the variant that sends it to the eighth is a setting.
• Spirits and markers. The eight spirits, the gate pressure, the punishment and grave rules follow the readings documented in the engine's design notes, and every rejection of an hour names its rule (how hours are chosen).

Why publish the choices

Because a rating that cannot be traced to a rule is just an opinion with a diagram. In the app every rating comes with “why I see this”: the rule, the signs and the threshold. The library explains the rules for a reader with no background and for a practitioner; the method page gives the numbers. If you follow a different school, the settings let you change the forks that are implemented, and the chart stores which ones were used.