Why Saju Calculators Differ: 5 Settings

Two saju calculators produce different four-pillar charts when they disagree on five settings: the year boundary (Ipchun vs. Lunar New Year), the exact solar-term minute for month pillars, the early-vs.-late Rat hour split at 23:00, clock time versus true solar time adjustment, and whether you entered a lunar or solar birth date.

Consider what happens when someone born in the weeks between Lunar New Year and Ipchun enters their birth date into two different apps. One app shows the new animal year because it counts from Lunar New Year; another still shows the previous animal year because it counts from Ipchun in early February. The instinct is to wonder which one to trust — but the more useful question is which year-boundary convention each tool applies. Every discrepancy flows from one of five calculable, verifiable settings. This guide explains each one.

Why Saju Demands Astronomical Precision

Saju (사주, Four Pillars of Destiny) assigns a heavenly stem and earthly branch to each of four time units: year, month, day, and hour. Those stems and branches are drawn from the sexagenary cycle and correspond to the position of the sun along its annual path — not to the social calendar most people use day to day.

This is the core tension. You live by a civil calendar, but saju is built on the 24-solar-term calendar (절기, jeolgi). The moment the sun enters a new degree of the ecliptic defines a new pillar — sometimes precise to the minute. A calculator that rounds to the nearest day and a calculator that uses the exact minute will produce different pillars for anyone born close to a boundary.

Classical Korean and Chinese scholars of fate-reading understood this. The cosmic forces they tracked do not pause for midnight or for the new moon.

Setting 1 — Year Boundary: Ipchun vs. Lunar New Year

The Two Schools

The most common mismatch for people born in January or early February is the year pillar. One app shows the Year of the Dragon; another shows the Year of the Snake. The explanation is direct.

One calculator uses Ipchun (입춘, Start of Spring), the first of the 24 solar terms, as the true start of the astrological year. Ipchun falls around February 4 each year. The other calculator uses the Lunar New Year, which falls anywhere from late January to mid-February.

Which Convention Is Standard?

Most classical Korean saju practitioners — and the majority of professional Four Pillars practitioners in China and Taiwan — use Ipchun. The rationale is internal consistency: every other pillar boundary in the chart is defined by solar terms, so using Lunar New Year for the year but solar terms for the months creates a structural contradiction.

Some popular apps default to Lunar New Year because it is culturally familiar to users. If you were born between Lunar New Year and Ipchun in any given year, these two systems assign you a different year pillar — which changes every ten-god (십신) relationship in your chart.

Setting 2 — Month Boundaries: The Exact Solar-Term Minute

Month Pillars Are Not Calendar Months

Each of the twelve month pillars is defined by the entry of the sun into a specific solar term. Gyeongchip (경칩, Awakening of Insects), which falls around March 6, begins the second month pillar of the saju year (the Rabbit month, after the Tiger month that opens at Ipchun). These entry times shift by a day or more from year to year and are accurate only to the hour and minute.

A calculator that switches month pillars at midnight on the calendar date will be wrong for anyone born on the day of a solar-term entry, before the exact minute. A calculator using an accurate solar-term ephemeris will match the classical standard.

The Practical Impact

If a solar term begins late in the evening and you were born a few minutes before that moment, you still belong to the earlier month, even though the calendar date has already changed. A calculator that switches months at 00:00 on the date of the term places you one month ahead, in the following month. That shifts your month stem and branch, affecting the ten-god relationships practitioners use for career, relationship, and luck cycle (대운, daewoon) analysis.

Setting 3 — The Rat Hour Split at 23:00

Late Rat vs. Early Rat

The twelve earthly branches divide the 24-hour day into twelve two-hour segments. The Rat hour (자시, jasi) runs from 23:00 to 00:59 and straddles midnight. Practitioners split it into two halves: late Rat (야자시, yajasi) from 23:00 to 23:59, and early Rat (조자시, jojasi) from 00:00 to 00:59. Every school gives early Rat to the new calendar day. The disagreement is only about the late-Rat half:

Day changes at 23:00: a birth between 23:00 and 23:59 takes the next calendar day's day pillar.
Day changes at 00:00: a birth between 23:00 and 23:59 keeps the current calendar day's day pillar.

Why This Is Decisive

If you were born in the last hour before midnight on a Monday, a calculator that changes the day at 23:00 assigns Tuesday's day pillar, and one that changes it at midnight keeps Monday's. The day pillar is one of your four core pillars and directly shapes the ten-gods that describe personality, relationships, and wealth patterns, including jeongje (정재, direct wealth), pyeonje (편재, indirect wealth), sikshin (식신, eating god), and sangwan (상관, hurting officer).

Neither convention is universally correct. Different lineages have maintained both positions for centuries. What matters is knowing which one your calculator uses, because the two will disagree on every birth time between 23:00 and 23:59.

Setting 4 — Time Basis: Clock Time vs. True Solar Time

What Is True Solar Time?

Standard clock time is a bureaucratic convenience. Every location within a time zone shares the same clock, but the sun does not reach its zenith at noon everywhere in that zone simultaneously. True solar time correction (진태양시 보정, jintaeyangsi) adjusts for two factors:

Geographic longitude relative to the time zone meridian. Korea Standard Time (UTC+9) is anchored at 135 degrees East, but Seoul sits at roughly 127 degrees East. That 8-degree gap translates to approximately 32 minutes of adjustment.
The equation of time — a small annual variation caused by Earth's elliptical orbit and axial tilt.

How Large Is the Difference in Practice?

For someone born in western South Korea, the longitude adjustment alone can exceed 30 minutes. A birth recorded just after 13:00 KST on the clock can fall before 13:00 in true solar time. That gap is usually inconsequential — but if the clock-time birth falls near a two-hour branch boundary such as 13:00, 15:00, or 17:00, the correction can push your hour pillar into the previous branch, changing your hour-pillar ten-god structure entirely.

A calculator that does not ask for your birth city or coordinates cannot apply a location adjustment, so it is working from raw clock time.

Setting 5 — Calendar Input: Lunar vs. Solar and the Leap Month Trap

The Calendar Confusion

Korea used the lunisolar calendar officially until 1896. Many families, particularly when birth dates were recorded by older relatives, still track birthdays in the lunar calendar (음력, eumnyeok). A child born on the 15th day of the 7th lunar month has a very different solar date depending on the year.

Entering a lunar date into a calculator that expects a solar date — or the reverse — makes every single pillar wrong. This is one of the most common sources of error, particularly for users with overseas Korean heritage or births recorded in older family registers (호적, hojeok).

The Leap Month Problem

The lunar calendar adds an intercalary (leap) month every few years to stay aligned with the solar year, effectively duplicating one lunar month. If you were born in a leap month — for example, a leap 7th month — and you enter the date as a regular 7th month without flagging it as the leap version, the solar conversion lands approximately 29 days off. Every pillar in your chart is built on the wrong date.

Most careful calculators provide a leap month checkbox or dropdown. Many casual apps do not. For anyone born in a leap month, this single omission corrupts the entire chart.

Why Practitioners Still Disagree Even With Identical Birth Data

Imagine two experienced saju practitioners given the same birth date, the same recorded time, and the same location. They can still produce different charts because one uses Ipchun for the year boundary while the other uses Lunar New Year, one uses true solar time while the other uses clock time, and they change the day at different times (23:00 or 00:00) for births in the late-Rat hour.

This is not a flaw in saju as a system. It reflects centuries of parallel development across Korean, Chinese, Taiwanese, and diaspora lineages before digital tools made cross-comparison easy. A practitioner trained in one lineage may use Ipchun, a 00:00 day change and clock time. A practitioner trained in another may use Ipchun, a 23:00 day change and true solar time correction. Both are applying their school's conventions consistently — neither is making an error.

This is why the first question most professionals ask is not simply what your birth date is, but whether you have an exact recorded birth time and whether it comes from a birth certificate or from a family member's memory. The birth time is the most fragile variable in the entire chart, and every one of the five settings above amplifies any uncertainty in that time.

How to Get a Consistent Chart: A Practical Checklist

Before comparing results across two calculators or bringing data to a practitioner, verify these five points:

Year boundary: Does the tool use Ipchun (around February 4) or Lunar New Year? For standard classical saju, Ipchun is the conventional choice.
Month boundary: Are month pillars derived from precise solar-term entry times, or from calendar month numbers? Look for a solar-term table or ephemeris reference in the tool's documentation.
Rat hour convention: Does the tool change the day at 23:00 or at 00:00? This only affects births between 23:00 and 23:59, but for those births the setting is decisive.
Time basis: Does the tool ask for your birth city or coordinates? If not, it is using raw clock time. A location-aware calculator that adjusts for longitude gives a more accurate hour pillar.
Calendar system: Know whether your birth date is lunar or solar before entering it. If it is lunar, check whether you were born in a leap month and whether the tool supports that flag.

Once you know those five parameters for a given tool, you can compare two tools in a principled way and identify exactly which setting is driving any difference.