Does BaZi use my clock time or solar time?

The hour pillar uses true local solar time, which is the sun's time where you were born, not the time on the clock. Clock time is a zone-wide convention, so a birth far from the middle of its time zone can be more than an hour away from the sun. Seoul runs about 32 minutes behind its clock, and Urumqi runs more than two hours behind Beijing clock time.

What happens to a BaZi chart for a birth after 23:00?

The traditional day begins at 23:00, not at midnight, so a birth between 23:00 and midnight falls in the rat hour of the following day and the day pillar rolls forward. This is the mainstream convention and it is the one Gilsung uses. A minority of teachers use a late-rat variant that keeps the calendar day, so this is the one difference of the four where two careful apps can honestly disagree.

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Why does my BaZi chart differ between apps?

Q: Why does my BaZi chart differ between apps?

Almost always because the two apps use different calendar rules, not because one of them is broken. Four differences cause nearly all of it: whether the year starts at lichun or at Lunar New Year, whether months follow the solar terms or the calendar, whether the hour uses clock time or true local solar time, and what happens to a birth after 23:00. Which pillar disagrees tells you which rule they disagree about.

Q: How do I tell which BaZi calculator is right?

Ask each one what it used. A calculator that states its rules can be checked, and one that does not cannot. Then test it on a birth date that sits between lichun and Lunar New Year, look up the exact time of the solar term at an observatory site, and compare the raw pillars rather than the reading.

Published 10 August 2026 by Pacefyx, Inc. We make Gilsung, a Four Pillars app, so we have a stake in this. Everything below can be checked without installing anything.

Because the two apps use different calendar rules, not because one of them is broken. Four rules cause nearly every disagreement. Which pillar differs tells you which rule they disagree about.

Your Four Pillars chart is arithmetic. Given the same rules and the same birth details, every calculator on earth returns the same eight characters. So when two of them differ, one of them is using a different rule, and you can find out which in a couple of minutes.

Which pillar is wrong, and what that tells you

The pillar that differs	Almost certainly this	Check
Year	One app starts the year at lichun. The other starts it at Lunar New Year.	Were you born between 20 January and 6 February?
Month	One app changes month at the solar terms. The other uses the lunar month or the calendar month.	Were you born within two days of the 4th to the 8th of a month?
Hour	One app converts to true local solar time. The other uses the clock time you typed in.	How far is your birth city from the middle of its time zone?
Day	One app rolls the day at 23:00. The other rolls it at midnight.	Were you born between 23:00 and midnight?

If two pillars differ at once, work through the causes in this order. The year and the day are the quickest to settle.

Does the BaZi year start at Lunar New Year?

No. The BaZi year starts at lichun, the solar term that begins spring, on 3, 4 or 5 February. Lunar New Year is a different thing on a different calendar, and it moves between 21 January and 20 February.

This is the single most common reason two charts disagree, and it is easy to see why. Almost everything else in popular Chinese and Korean culture, the animal signs on greeting cards included, is counted from Lunar New Year. The Four Pillars year is counted from the sun.

So there is a gap of a few days to a few weeks each year where the two answers differ. If you were born in that gap, one app gives you last year's animal and the other gives you this year's, and only one of them is doing what the tradition does.

A test case you can run on any calculator

Take a birth on 10 February 1996. Lunar New Year 1996 fell on 19 February, but lichun fell on 4 February, so that birth is already in the new year by the rule the tradition uses.

The correct year pillar is 丙子, fire rat. A calculator anchored to Lunar New Year will give you 乙亥, wood pig, which is the year before. Type that date into any two calculators and you will see which rule each one follows.

Lichun has a time of day, not just a date. It is an exact moment when the sun reaches a fixed point, and it can land at 21:08 in the evening. A birth earlier the same day is still in the old year. Observatories publish these moments to the minute, which is why the good calculators agree to the minute.

What to check in your own chart: if you were born between 20 January and 6 February, look at the year pillar and compare it to the animal your family says you are. If they differ, this is why, and the chart is not necessarily the thing that is wrong.

Why do BaZi months not match calendar months?

Because the month pillar changes at the twelve sectional solar terms, not on the first of the month and not at the new moon. Each solar term is a fixed point on the sun's path, so the dates move by a day or two from year to year.

The word for the calendar here is a real trap in English. The Chinese calendar has lunar months, and BaZi does not use them. It uses the solar half of the same calendar, the twenty-four terms that farmers used for planting. An app that reads your birth date as a lunar date will get your month pillar wrong most of the year.

Month branch	Begins at	Usually
寅 tiger	lichun 立春	3 to 5 February
卯 rabbit	jingzhe 驚蟄	5 to 7 March
辰 dragon	qingming 清明	4 to 6 April
巳 snake	lixia 立夏	5 to 7 May
午 horse	mangzhong 芒種	5 to 7 June
未 goat	xiaoshu 小暑	6 to 8 July
申 monkey	liqiu 立秋	7 to 9 August
酉 rooster	bailu 白露	7 to 9 September
戌 dog	hanlu 寒露	7 to 9 October
亥 pig	lidong 立冬	6 to 8 November
子 rat	daxue 大雪	6 to 8 December
丑 ox	xiaohan 小寒	5 to 7 January

What to check in your own chart: find your birth date in the table. If you were born within two days of one of those windows, your month pillar depends on the exact hour the term began. The Hong Kong Observatory and the National Astronomical Observatory of Japan both publish those hours, and they are linked at the bottom of this page.

Why does BaZi use solar time instead of clock time?

Because the hour pillar is about where the sun was, and clock time is a political convention. A time zone is one hour wide in theory and often much wider in practice, so the clock on your birth certificate can be a long way from the sun over your birth city.

The arithmetic is simple. Fifteen degrees of longitude is one hour, so one degree is four minutes. Look up how far your birth city sits from the meridian its clock is set to, and that is your correction.

Seoul sits near 127 degrees east while Korean clocks are set to 135 degrees east. The sun over Seoul is about 32 minutes behind the clock.
Urumqi sits near 88 degrees east while the whole of China runs on one zone set to 120 degrees east. The gap is more than two hours.
Madrid sits west of Greenwich while Spain runs on central European time. The sun there is about 75 minutes behind the clock.

True local solar time adds one more small correction on top. The earth's orbit is not a perfect circle, so the sun runs up to about 16 minutes ahead of or behind the average across the year. That is the difference between mean local time and true solar time, and near an hour boundary it decides your hour branch.

A worked example. A birth in Seoul at 07:20 on the clock is about 06:48 by the sun. The hour branches change every two hours, so 06:48 is still the rabbit hour 卯, while 07:20 would have been the dragon hour 辰. Same birth, different hour pillar, and the whole reading changes with it.

The summer time trap

Several countries used summer time in years people have forgotten, and an hour of unrecorded daylight saving moves an hour pillar as surely as a longitude correction does.

China used summer time from 1986 to 1991. Japan used it from 1948 to 1951. South Korea used it in three separate stretches: 1948 to 1951, 1955 to 1960, and 1987 to 1988. If you or a parent were born in one of those windows, check the offset before trusting an hour pillar.

Korea is the one that catches people out, because two corrections overlap there. Korean standard time ran half an hour behind today's setting from 1954 to 1961, and summer time sat on top of that from 1955 to 1960. A Seoul birth in July 1958 was on a clock 30 minutes ahead of today's Korean time, not an hour, and not the half hour behind you would expect from the standard offset alone.

The practical version: someone born in Seoul at 14:00 on 10 July 1958 was born at 04:30 UTC. An app that assumes Korea has always been nine hours ahead puts that at 05:00 UTC and is half an hour out, which is enough to move an hour pillar if the birth was near a boundary.

What to check in your own chart: work out your correction in minutes. If your birth time is within that many minutes of an odd hour on the clock, 01:00, 03:00, 05:00 and so on, then your hour pillar is genuinely borderline and two apps will disagree about it.

What happens to a chart for a birth after 23:00?

The traditional day begins at 23:00, not at midnight, so a birth between 23:00 and midnight belongs to the next day's day pillar. The rat hour 子 straddles midnight, and the day turns at its start.

A birth at 23:40 on 3 May therefore carries the day pillar of 4 May. An app that rolls the day at midnight will give you the pillar for 3 May, and every reading built on the day master will be about a different person.

This is the one difference of the four where two careful practitioners can honestly disagree. Most schools roll the day at 23:00. A minority use a late-rat reading that keeps the calendar day and changes only how the hour stem is worked out. Gilsung rolls the day, which is the mainstream convention, and says so.

What to check in your own chart: if you were born in that last hour of the day, look at whether the day pillar matches your birth date or the day after. Then look at whether the app tells you which rule it applied. That second part matters more than the first.

How do I tell which BaZi calculator is right?

Ask each one what it used. A calculator that states its rules can be checked. A calculator that does not state them cannot be checked, and it is asking you to trust arithmetic you are not allowed to see.

Run the boundary test. Enter 10 February 1996. A calculator that returns the wood pig year is anchored to Lunar New Year.
Compare pillars, not readings. Two apps can write very different paragraphs from identical pillars. Line up the eight characters first.
Check the hour rule. Enter a birth city far from its zone's meridian and see whether the hour branch moves. If it never moves, the app is using clock time.
Look up the solar term yourself. The observatory tables linked below settle a month pillar in about thirty seconds.

Given the same rules and the same birth details, every Four Pillars calculator returns the same eight characters. A disagreement is always about the rules.

Gilsung, our own Four Pillars app, was built around that sentence. It states all four conventions it used, and every reading has a control called "Show what this reading came from". Whichever tool you end up using, the one worth trusting is the one that will tell you its rules.

Common questions

Is my animal sign wrong if I was born in early February?

Your Four Pillars year branch may not be the animal you grew up being told, if you were born between 20 January and 6 February. The popular animal comes from Lunar New Year and the chart's comes from lichun. Both are real customs and they are counting different years.

Do I need my exact birth time for a BaZi chart?

You need it for the hour pillar, which is one of the four and the one that moves fastest. The year, month and day pillars are fine without it. If your time is recorded only to the nearest hour, treat the hour pillar as uncertain rather than dropping it.

Does the birth city matter, or only the country?

The city matters, because the hour pillar depends on longitude. Two births at the same clock time in the same country can sit in different hour branches if the cities are far apart east to west. A calculator that asks only for a country cannot get this right.

Which BaZi calculator should I trust?

The one that tells you which conventions it used, whichever that turns out to be. Rules you can read can be checked against an observatory table. Rules you cannot read leave you comparing two black boxes.