Birth time, solar terms and time zones

The solar terms set the year and month

The year in Saju begins at Ipchun (입춘 立春, the start of spring): the moment the Sun's apparent longitude reaches 315 degrees, around 4 February. From then on, every 30 degrees of the Sun's path opens a new month at one of twelve jeol (절): Ipchun, Gyeongchip, Cheongmyeong, Ipha, Mangjong, Soseo, Ipchu, Baengno, Hallo, Ipdong, Daeseol and Sohan.

We calculate each solar term from the Sun's position with an astronomical library, so the change is placed to the minute. We checked our times against the solar term tables published by the Hong Kong Observatory: every term from 2020 and 2024 to 2026 falls within a minute.

If you were born within a day of a solar term and do not know your time, your month pillar (and at Ipchun, your year pillar) could go either way. The calculator tells you when that happens.

The day and the hour

The day pillar comes from an unbroken count of sixty days that has run for many centuries. The hour pillar covers two hour blocks named after the twelve branches: the Rat hour 子 from 23:00 to 01:00, the Ox hour 丑 from 01:00 to 03:00, and so on. Its stem follows from the day stem.

That leaves one old question: when does the day change? By default we follow the widespread rule that the new day starts with the Rat hour at 23:00. Some Korean schools split the Rat hour: a birth between 23:00 and midnight keeps that day's pillar (야자시 yajasi, the night Rat hour), and a birth after midnight takes the next (조자시 jojasi, the early Rat hour). The hour pillar is the same either way; only the day pillar changes. You can choose in the calculator's options.

Clock time or solar time

Hour boundaries are about the Sun, and time zones are about convenience. Korea Standard Time is set for 135 degrees east, but Seoul sits near 127 degrees east, so the Sun reaches its highest point in Seoul around 12:32 on the clock. That is why Korean practice often places the Rat hour at 23:30 to 01:30 instead of 23:00 to 01:00.

The calculator gives you three choices. Clock time uses the time you entered (with daylight saving removed). Local mean solar time corrects for the longitude of your birthplace: about 32 minutes earlier in Seoul, and more in the west of big time zones. True solar time also adds the equation of time, the wobble between the Sun and the clock that reaches about 16 minutes in early November and 14 minutes in February. We default to local mean solar time when you pick a place, which matches common Korean practice.

Korea's changing clocks

Korea's official time has changed several times. From 1908 to 1911 clocks ran on UTC+8:30. From 1912 they ran on UTC+9. From 21 March 1954 to 10 August 1961 South Korea went back to UTC+8:30, and it has used UTC+9 since. There was summer time (an hour ahead) in 1948 to 1951, 1955 to 1960, and 1987 to 1988, the Seoul Olympics years.

A birth time written on a certificate is clock time, so these changes matter. The calculator uses the IANA time zone database, the same history your phone uses, to turn a clock time into the true moment of birth, and tells you when your date fell in a UTC+8:30 or summer time period. Births outside Korea get the same treatment for their own zones.

Unknown time

Plenty of people do not know their birth time. You still get three pillars, six characters and the day master. Leave the time empty and the chart simply shows the hour pillar as unknown.

When each Saju year starts

Ipchun, the start of the Saju year, in Korea Standard Time, calculated from the Sun's position.

Questions

Work out your four pillars