Place and Time of Birth: Longitude, Daylight Saving and the Ja Hour

Clock noon and solar noon

Saju time is based on the sun. Noon is when the sun reaches its highest point in the sky (culmination), and the twelve double-hours (시진) divide the day into twelve two-hour slots. The clock we use, however, runs on a standard time that the whole country shares. Korea Standard Time is set to 135°E, that is, to the sun over Japan.

The Earth turns 360 degrees a day, so for every degree of longitude the sun reaches its highest point 4 minutes later. Seoul lies at about 127°E, roughly 8 degrees west of 135°, so the sun culminates about 32 minutes late. When the clock in Seoul reads 12:00, the sun's time is still around 11:30. Even within one country, the gap shrinks the further east you go and grows the further west you go. The table below is calculated from the longitudes of the cities on Halmeoni's list.

Double-hour boundaries come every two hours, so even a correction of around 30 minutes is large enough to change the hour pillar (시주) of someone born near a boundary. That is why Halmeoni asks for your city of birth. The city is used only to correct the time; Halmeoni does not ask for your device's location.

The correction applies only to the day and hour pillars

The longitude correction does not go into every pillar. The year pillar (연주) and the month pillar (월주) are set by comparing the moment a solar term (절기) begins, to the minute, with the moment of birth. A solar term arrives at the same single instant everywhere in the world, so the moment of birth is converted to Coordinated Universal Time (UTC) and compared as it is, with no longitude correction. The day pillar (일주) and the hour pillar, by contrast, are based on "how the day passed in that place," so they are read in the sun time of the birthplace (the corrected time).

If you do not know your birth time, however, the day pillar uses the date exactly as entered, so this correction is not applied. This is explained further in "When you don't know your birth time" below.

Schools correct the time in different ways

• No correction: the clock time on the birth record is used as it is. This is the easiest calculation, but for a birth in Seoul it sets the hour pillar with a time more than 30 minutes ahead of the sun.
• A flat 30-minute correction: 30 minutes are subtracted wherever in Korea the person was born. It is simple, but it ignores the difference between Busan and Mokpo.
• Longitude correction by city (local mean time, 지방평균시): calculated from the longitude of the birthplace at 4 minutes per degree.
• Correction including the equation of time (true solar time, 진태양시): on top of the longitude correction, the amount by which the sun's culmination swings earlier or later through the year (the equation of time, 균시차) is added or subtracted again, date by date.

Here is how Halmeoni reads it. The default is the longitude correction by city. About 32 minutes are subtracted for Seoul and about 24 minutes for Busan, so each city is calculated differently. The equation of time is not included by default. The equation of time is a difference that arises because the Earth's orbit is elliptical and its axis is tilted; in Halmeoni's formula it moves between roughly −14 and +16 minutes over a year. Some manseryeok (만세력) use true solar time, so if you were born within about ten minutes of a double-hour boundary, your hour pillar may come out differently elsewhere.

As one example, for someone born in Seoul on 3 November 2023, at 11:25 on the clock, the time corrected for longitude alone is 10:52, which is the Snake hour (巳時 사시); with the equation of time as well it is 11:09, which is the Horse hour (午時 오시). Early November is when the equation of time is at its largest in the year.

Example: the same time, Seoul and Busan

Consider two people born on 20 May 1990, at 1:30 pm (13:30) by the clock. One was born in Seoul, the other in Busan. Their year, month and day pillars are the same: 庚午 경오 year, 辛巳 신사 month and 乙酉 을유 day. What differs is the hour pillar.

In Seoul, 32.1 minutes are subtracted to give 12:57 (seconds dropped), which falls at the very end of the Horse hour (오시, 11:00–13:00). In Busan only 23.7 minutes are subtracted, giving 13:06, the Goat hour (미시, 13:00–15:00). Although the two were born at the same moment, both the top character and the bottom character of the hour pillar differ. Even for the same Seoul birth, a manseryeok that applies no correction, or one that also includes the equation of time, gives the Goat hour. If your hour pillar comes out differently from another source, check the correction method first.

Korea Standard Time has changed several times

Korea has not always used the 135°E time. Halmeoni first converts the clock time to universal time using the standard time recorded for that period in the international time zone data (IANA), and then applies the longitude correction. So for the same Seoul birth, the amount subtracted differs depending on the year of birth.

The UTC+8:30 used from 1954 to 1961 is based on 127.5°E, so it nearly matches the sun in Seoul. A Seoul birth in this period needs a correction of only a little over 2 minutes. If you were unaware of this period and subtracted a flat 30 minutes, the hour pillar could be thrown off. Before 1908, the time zone data treats the time as Seoul local mean time, so a Seoul birth needs almost no correction.

Daylight saving time

Daylight saving time is a system that moves the clock one hour ahead during the summer. If you were born in that period, the time on the birth record is one hour ahead of the sun's time. The time zone data Halmeoni uses contains Korea's daylight saving time for the twelve years below. The start is when the clock was moved one hour forward, and the end is when it was moved one hour back, at that time on the daylight saving clock.

From 1955 to 1960, one hour of daylight saving time was added on top of UTC+8:30, giving UTC+9:30. So even for the same daylight saving time, a Seoul birth in the summer of 1988 has 92 minutes subtracted, and one in the summer of 1958 has 62 minutes subtracted. Halmeoni does this calculation automatically, so enter the clock time written on the birth record as it is. If you subtract an hour in advance, it is subtracted twice.

On the day daylight saving time starts, there is an hour that the clock skipped. If you enter a time that did not actually exist, such as 02:30 on 10 May 1987, Halmeoni reads it as standard time and writes on the saju card, "This time was skipped by daylight saving time, so I calculated with standard time." On the day it ends, the same time passes twice; in that case Halmeoni calculates with the earlier time (the daylight saving one) and tells you so on the card.

If you were born abroad

When you choose the country and city of birth, Halmeoni uses the time zone there (including the standard time and daylight saving records of that period) and the longitude in the same way. Within one time zone the correction still varies with longitude, so the city name matters. If your place is not on the list, choose a nearby city in the same time zone. One degree of longitude is about 90 km at Korea's latitude and equals 4 minutes, so a difference of a few tens of kilometres is around 1 to 2 minutes. Unless you are near a double-hour boundary, the result does not change.

11 pm to 1 am: the Night Ja hour and the Early Ja hour

The Ja hour (子時) runs from 23:00 to 1:00, so it straddles midnight. There are therefore two methods for deciding when the date changes. One treats the day as already the next day once the Ja hour begins at 23:00 (the whole-Ja-hour method, 정자시 or 통자시). The other calls 23:00 to midnight the Night Ja hour (야자시), which keeps that day's day pillar, and midnight to 1:00 the Early Ja hour (조자시), which uses the new day's day pillar. In both methods the hour pillar from 23:00 on is set according to the next day's day stem, so only the day pillar differs.

Here is how Halmeoni reads it. The default is the method that separates the Night Ja hour and the Early Ja hour, so the day pillar changes at midnight. Midnight here is not the clock's midnight but midnight in the corrected time. In Seoul, up to about 00:32 on the clock it is still the previous night.

Look at the second row. By the clock it is already the 21st, but the corrected time is 23:47 on the 20th, so Halmeoni uses the day pillar of the 20th. In a manseryeok that uses the whole-Ja-hour method, the same person's day pillar comes out one day later. When the Day Master changes, the Ten Gods (십신) and the whole reading change, so if you were born in this window, be aware that your result may differ from other sources.

When you don't know your birth time

If you mark the time as unknown, Halmeoni leaves out the hour pillar and reads three pillars, the year, month and day pillars, six characters in all. The day pillar is the date exactly as entered, and Halmeoni does not offer interpretations that depend on the hour pillar (such as those about children or later life). The saju card says, "You didn't know the birth time, so I read it without the hour pillar."

The year and month pillars are set by comparing the time of birth with the exact moment, to the minute, at which a solar term begins; if the time is unknown, noon of that day is used as the reference. So if you were born on a day when a solar term changes, the year or month pillar may be wrong. For example, Ipchun (입춘) began at 17:27 on 4 February 2024. If you enter this day without a time, it is treated as noon, giving the 癸卯 year and 乙丑 month; but if you were born at 17:40, it is the 甲辰 year and 丙寅 month, and both the zodiac animal and the month change. On such days Halmeoni also shows the notice, "Your birthday overlaps with a solar term."

If you do not know the exact time but know roughly when, such as "around four in the morning," you can try entering a time so that the corrected time falls near the middle of that double-hour. For a Seoul birth, that is the clock time of the middle of the double-hour plus 32 minutes (for example, 4:32 for the Tiger hour 寅時 인시). If you are still close to a double-hour boundary, enter both of the neighbouring double-hours and compare the results.

How to check for yourself

• The time written on a birth certificate or medical record is the most reliable source. However, if the time is close to the boundary of two double-hours or to a solar term, a difference of a few minutes can change the result.
• If you were born in the 1950s–60s or in the late 1980s, check whether it was a daylight saving period, but enter the time exactly as the clock time written on the record.
• If the result differs from another manseryeok, first compare the longitude correction method, whether the equation of time is included, and how the Ja hour is split. In many cases neither is wrong; the standards are simply different.