Why do saju sites give different month pillars for births near a solar term?

The month pillar changes at the solar term moment

The saju month changes at 12 solar terms, not on the first of the calendar month. In 2027, Gyeongchip (Awakening of Insects) falls on March 6 at 4:40 a.m. Korea Standard Time. A birth before that minute is in the Tiger month (寅), at or after it in the Rabbit month (卯).

One minute across the line changes both characters of the month pillar, and the reading built on that pillar changes with it.

Three reasons for the difference

The first is the source of the solar term times. Some services use the values published by KASI, others their own astronomical calculation or values from printed almanacs. The methods can differ by several minutes.

The second is longitude correction. Korea Standard Time is based on 135 degrees east, but Seoul sits near 127 degrees, so by the sun's actual position Seoul runs about 32 minutes behind the clock. Some services apply this and some do not.

The third is daylight saving time. Korea used it in 1948 to 1951, 1955 to 1960, and 1987 to 1988. Taking a birth time from those years at face value puts it an hour off.

How SAJUDORIM handles it

SAJUDORIM uses KASI's published values, to the minute, for the solar terms of 2000 to 2028. For other years it uses an astronomical calculation that differs from published values by up to about 12 minutes.

In those years, if a birth falls within 12 minutes of a solar term, the result screen says another service may give a different month pillar. Choosing a birthplace applies the longitude correction, and births during daylight saving time are calculated with the standard time in force then.