How we calculate: methods and sources

When the same birthday gives different Saju results on different sites, it is usually because they handle clock time differently. Here is exactly what we calculate and how, with nothing hidden.

Day Pillar

The day cycle repeats every 60 days, and we count it straight from the calendar date. The count is anchored on January 1, 2000, which was a Mu-o (戊午) day.

Year and Month Pillars

In Saju, the year changes at Ipchun (Start of Spring), not at Lunar New Year, and each month changes at a solar term (Ipchun, Gyeongchip, Cheongmyeong, and so on). We find each solar term by computing the Sun's position with an astronomical formula (VSOP87, from Meeus's Astronomical Algorithms). Compared with 40 equinox and solstice times published by the US Naval Observatory (USNO), every one matches within 1 minute, and the 24 solar terms of 2027 also match published times within 1 minute.

Korea's historical time zones

This table applies to birthplaces in Korea.

Korea observed daylight saving time from 1948 to 1951, 1955 to 1960, and 1987 to 1988. The rules for each date follow the IANA time zone database (Asia/Seoul). Some years differ slightly between sources, so we cross-check against official records. For birthplaces outside Korea, we use the browser's IANA time zone data for that city on your birth date, including daylight saving time.

Birthplace

A time zone is built around a single meridian, but cities sit east or west of it. When the clock says noon, the Sun may not yet be due south, or may already have passed. We correct for your birthplace's longitude (local mean solar time) before choosing your Hour Pillar.

Cities differ by only a few minutes, so your result changes only if you were born close to an hour boundary. When that happens, the result screen tells you.

Around midnight

The Rat hour (jasi) runs from 11 PM to 1 AM. Some traditions say the day changes at midnight (yajasi), and others say it changes at 11 PM (jasi). We let you choose, and the default is yajasi.

Lunar birthdays

Entering a birthday on the lunar calendar is available only on the Korean version of this site. There, the date is converted to the solar calendar and calculated the same way. Saju changes years and months by solar terms, not by the lunar calendar, so lunar dates are used only for that conversion.

What you enter

All calculation runs inside the browser on your own device. Your birthday and time are never sent to a server or stored.

The readings

A character's name combines the color (element) of the Day Pillar's Heavenly Stem with the animal of its Earthly Branch. The readings are reviewed by a person, and text written with help from AI is labeled as such. Fortunes here are just for fun.