Guide
Why do BaZi calculators disagree?
Four honest reasons two calculators can give you different pillars, and the one question that tells you whether it matters for you.
BaZi calculators disagree when they make different choices about time: whether to correct for true solar time, where the day begins, and where the year begins. Births near a boundary are the ones that change. Everyone else gets the same chart everywhere.
It's about time, not about BaZi
Put the same birth data into five calculators and you can get two or three different charts back. It feels like the system is unreliable. It isn't. The system is nearly identical everywhere. What differs is how each calculator interprets your birth time.
BaZi is a time system wearing an astrology costume. Every pillar is a function of when, exactly, you were born. So every judgment call a calculator makes about time becomes a judgment call about your chart. There are four of them, and they explain nearly every disagreement.
1. Clock time or sun time
Your wall clock says 2:00. The sun may disagree. Traditional BaZi runs on true local solar time: the time told by the sun where you were actually born, not the time your city's clocks observe. Two corrections go into it. Longitude: every degree east or west of your time zone's center line shifts solar noon by four minutes. And the equation of time: a seasonal wobble, up to about sixteen minutes, from the Earth's tilted orbit.
Many online calculators skip both and feed your clock time straight in. Near an hour boundary, that flips the hour pillar. Near midnight, it can flip the day.
When you give Aster your birthplace, the engine applies both corrections. That is the traditional practice, and it is stated here so you can hold us to it.
2. Where the day begins
This one is a genuine disagreement between schools, not a bug. The zi hour runs from 11pm to 1am, straddling midnight. Some calculators start the new day at midnight, the way your phone does. Others follow the older convention: anything born from 11pm onward belongs to the next day, because the zi hour belongs to the new day.
A birth at 11:30pm gets a different day pillar, and a different Day Master, depending on which convention the calculator follows. This is the single biggest source of "my Day Master changed" surprises.
Aster rolls the day forward at 23:00. We consider it the more defensible reading of the tradition, and we state it plainly so you can compare.
3. Where the year begins
The BaZi year does not begin on January 1. It begins at Start of Spring, around February 4. Anyone born in January or the first days of February belongs, in BaZi terms, to the previous year: different year pillar, different animal, different chart weather.
Serious calculators all do this. Cheap ones sometimes use January 1, and every January baby gets the wrong year pillar. If a calculator handed you the animal of the calendar year you were born in and you were born in January, be suspicious.
(The month pillars work the same way: they turn at the solar terms, not on the first of the month. On this one the good calculators agree.)
4. Daylight saving
Was daylight saving in effect where and when you were born? An hour's difference near a pillar boundary changes the chart, and some calculators never ask. This one is pure bookkeeping, but it bites.
The one question that matters
Here is the reassurance, and it is real: if you were born at 2pm on an ordinary Tuesday in June, every competent calculator on earth gives you the same chart. Disagreement lives at the edges. So the only question you need to ask is: was I born near a boundary?
Near midnight. On a solar-term day. In January or early February. On a daylight-saving switchover. If none of those apply, stop worrying and pick the calculator you like. If one does, check which convention it follows, and prefer the one that tells you.
That last clause is the whole test. A calculator that states its conventions can be argued with. One that doesn't is a black box with nice fonts.
What Aster does, stated plainly
So you can compare: Aster corrects clock time to true solar time when it knows your birthplace. It rolls the day forward at 23:00. It starts the year at Start of Spring and the months at the solar terms. When birth time is unknown, it computes from local noon, the point furthest from both day boundaries, and leaves the hour pillar out rather than inventing one.
No hidden choices. If you disagree with a convention, you know exactly which one to argue with.
Disagreement lives at the edges. Born near a boundary, check the conventions. Otherwise, every honest calculator agrees.