The Day Master (日主): the one stem every label is measured against
How it is obtained
The day pillar comes from the sexagenary day count, and its stem (not its branch) is the Day Master. In this chart — born 20 May 1990, 辰 slot — the day pillar is 乙酉, so the Day Master is 乙.
| Pillar | Stem / Branch |
|---|---|
| Year | 庚午 |
| Month | 辛巳 |
| Day | 乙酉 ← Day Master = 乙 |
| Hour | 庚辰 |
What can change it
- Nothing inside the chart. The Day Master is an input to every other label, never an output — it is not "strengthened" or "weakened" into being something else.
- The birth date can. A birth close to midnight can land on either side of the day rollover, which changes the day pillar and therefore the Day Master. Our convention: a 23:00–24:00 birth takes the next day's pillar (early-Zi school) — see the zi hour.
Why the whole notation hangs off it
The Ten Gods are not properties of stems; they are properties of relations. The same stem gets a different label depending on which Day Master it is compared with, which is why the same character can be Rival (劫財) in one chart and Rob Wealth in another. The ten labels and how they are derived are in The Ten Gods explained.
What the Day Master does not tell you
Knowing it is 甲 or 辛 tells you the reference point, not the person. Strength assessments, structure (格局) and any prediction sit outside what this site computes — the engine outputs the pillars, the labels and the relations, and stops there.
Frequently asked questions
Is the Day Master the same as the “self element”?
Yes in common usage: the Day Master’s phase is called the self phase, and every other phase is named relative to it (output, wealth, officer, resource). Two charts with the same Day Master share that frame, though nothing else.
Can the Day Master change if I adjust the birth time?
It changes only if the day pillar changes — a birth near midnight or near a day-rollover boundary. Moving the hour within the same two-hour slot never changes it. The zi-hour page shows a worked case.
Does the Day Master predict personality?
Not here. It is a reference point for labelling relations. Traditional readings attach character traits to it; this site treats those as interpretative tradition, not as output of the computation.
Recompute it yourself
The same engine is published as yilibazi-core on npm (MIT), with source on GitHub — conventions exported as constants, and an ambiguityReport() that returns the alternative chart whenever a convention would change the answer. The stated rules and the known gaps are in the method page.