Why saju apps give different results

01

What did the clock say in Korea then?

The standard time and summer time Korea used on the day of the birth are applied as they were.

Korea used summer time in 1948–1960 and 1987–1988, and from 1954 to 1961 its standard time was UTC+8:30, half an hour behind today's.

So 10 a.m. in July 1987 is 9 a.m. on today's clock. Ignore it and the hour pillar moves, and near midnight the day pillar too. This site finds the day's offset in the international time zone data (IANA tz, old records included).

On this site"UTC offset used" under "How this was calculated" is this value. A birth at a summer-time change is flagged when you enter it.

02

Why take about 30 minutes off the birth time?

Korean standard time is set for 135° east, so in Seoul (about 127°) the sun runs about 32 minutes behind the clock.

The hour pillar is a two-hour slot of the sun's position. So this site reads it from the clock corrected for the birthplace's longitude (local mean time): -32 minutes in Seoul, -24 in Busan.

Some apps skip this correction, so a birth near an odd hour gets a different hour pillar from app to app. Some go further and correct for the equation of time (true solar time); this site uses local mean time.

On this site"Time correction" under "How this was calculated" gives your birthplace's minutes. Within 15 minutes of a slot boundary, the result also names the neighbouring hour pillar.

03

Born after 11 p.m.: which day is it?

Corrected 23:00–24:00 keeps that day's day pillar; only the hour pillar's stem is taken from the next day.

The Rat hour runs from 23:00 to 01:00. Some schools change the day at 23:00, others at midnight. This site changes it at midnight by default (the split between late and early Rat hour).

If you were born in this hour, you are asked which reading to use when you enter it, and the choice is shown under "How this was calculated".

On this site"Rat hour rule" under "How this was calculated" is this choice.

04

When do the saju year and month change?

The year changes at Ipchun (the start of spring), not on 1 January or the lunar new year, and each month at a solar term, not on the 1st.

Saju months start at 12 of the 24 solar terms, which divide the sun's path (Ipchun, Gyeongchip, Cheongmyeong …). The year pillar also changes at the moment of Ipchun.

So someone born in January or early February has the previous year's zodiac animal and year pillar. A birth within an hour of a solar term is flagged when you enter it. The term times come from an astronomical library (lunar-javascript).

On this siteYour Year's months are these solar-term months, so "November" starts at Ipdong, around 7 November.

05

Why does the Useful God differ between apps?

There are several ways to choose it, and this site uses strength balancing (eokbu) by default.

The Useful God is the element that balances a chart. Methods such as strength balancing, climate (johu), mediation, illness-and-medicine and structure can give the same chart different Useful Gods, and then the "good years" can be the opposite.

This site splits strong, balanced and weak by the share of support for the day master (Companions and Resource; 0.50 and up is strong, 0.35 and below weak) and picks the Useful God from a fixed balancing table. Charts from deep winter or high summer add a climate element, and a follow structure is used only under strict conditions. Mediation, illness-and-medicine and structure methods are not used: they rest on the reader's judgement and cannot be applied the same way to everyone in code.

On this site"Useful God" under "How this was calculated" gives your chart's Useful God and why, in one line. The yearly graph's scores stand on it.

06

What is the calculation based on?

Public data and general saju principles, with every rule stated on this page and under "How this was calculated".

If another app still disagrees, check its time correction and Rat-hour rule first. That is where most differences come from.