The calculation layer — differences with a right answer

The year and month pillars change at the exact moment a solar term begins — not on January 1 and not on Lunar New Year. If Ipchun arrives on the morning of February 4, someone born before dawn that day still belongs to the previous year's animal. Apps that only compare dates get the year and month pillars wrong for people born on those boundary days.

Korean standard time has not always been the same. From 1908 to 1911 and again from 1954 to 1961 the country used UTC+8:30, thirty minutes behind today, and it ran daylight saving time in several years between 1948 and 1960 and again in 1987–1988. Take a birth time from those periods at face value and the hour pillar can slip by a full slot.

True solar time is handled differently too. Korean clocks run on the 135°E meridian, while Seoul sits near 127°E, so the clock is about half an hour ahead of the sun. Some apps subtract a fixed 30 minutes, some use the birth city's longitude, and some also add the equation of time. For a birth time near an hour boundary, that choice alone can change the hour pillar.

Lunar birthdays add another gap. Korea and China define the new moon from clocks an hour apart, so in some years Lunar New Year falls on a different day (1988, for example) and the leap month lands in a different month (2012, 2017). Feed a Korean lunar birthday into a tool built on the Chinese table and the date can shift by a day or even a month.

The interpretation layer — where schools disagree

For people born between 11 p.m. and midnight, schools disagree on whether the day pillar belongs to that day or the next. How many points make a chart "strong," whether the useful element follows balance or season, whether yin stems count the twelve life stages backwards, whether peach-blossom and travel stars are read from the year or the day branch — all long-running debates. This layer has no single right answer. What it can have is disclosure.

The rules ARO uses

Solar terms: a 1900–2050 table computed from the sun's ecliptic longitude to the minute, cross-checked against 144 Hong Kong Observatory solar-term times for 2017–2028, all within one minute. Time zones and daylight saving: the IANA time zone database's Korean history, including UTC+8:30 in 1954–1961 and the exact DST start and end times. True solar time: four minutes per degree of the birth place's longitude, without the equation of time. Lunar dates: converted with the Korean lunar table based on the Korea Astronomy and Space Science Institute.

Births between 11 p.m. and midnight: the day pillar stays on that day, and only the hour stem follows the next day (the ya-jasi convention). If the birth time is unknown, ARO leaves the hour pillar empty and reads the other three, because a guessed hour would ripple into every reading that depends on it.

Interpretation: strength weighs the month branch most, then the day branch, then the remaining positions, and is read as a score. The useful element follows the balance (eokbu) method, the twelve life stages count yin stems in reverse, and the void (gongmang) is taken from the day pillar. The twelve stars are shown from both the year and the day branch. Luck cycles run forward or backward by the year stem's polarity and your gender, and their starting age converts the actual time to the next or previous solar term at three days to one year and one day to four months.

So if another app disagrees with ARO, check which of these rules differs. A difference in the calculation layer means one side made an error; a difference in the interpretation layer means the two are following different schools.

Related terms

• 장생 (長生)The starting stage — newly born and reaching out. Strong energy of beginnings and growth.
• 비겁A rhythm of activity and rallying together.
• 인성Being-cared-for mode. Rest comes easy.