Calculation Standards
If I was born between 11 p.m. and 1 a.m. (the Rat hour), which day is my Day Pillar?
As of · Edited by Sajunyang · about 1700 characters
One-line answer
The Rat hour (子時) runs from 11 p.m. to 1 a.m. The traditional method (the standard Rat hour) treats 11 p.m. as the start of the next day and changes the Day Pillar there. The late Rat hour (夜子時) theory keeps the current day's Day Pillar for 11 p.m. to midnight while attaching the next day's Rat hour as the Hour Pillar, and treats the time after midnight as the early Rat hour (朝子時) belonging to the next day. The Day Pillar differs by a day depending on which method is used, so people born in the Rat hour should check both results.
How do the standard Rat hour and the late Rat hour differ?
The standard Rat hour (正子時) method changes the day at the start of the Rat hour, that is, at 11 p.m. For a birth at 11:30 p.m. on October 24, the Day Pillar is the stem and branch of October 25. The late Rat hour theory changes the day at midnight but treats only the Hour Pillar as the next day's Rat hour. For the same birth, the Day Pillar is the stem and branch of October 24, and the Hour Pillar is the Rat hour stem based on the 25th's Day Master. A birth at 12:30 a.m., after midnight, gets the 25th's Day Pillar under both methods.
Which one is correct is still debated in Saju circles. The classics use the standard Rat hour, and some modern schools use the late Rat hour. Sajunyang sets the standard Rat hour as the default and lets you select the late Rat hour in the advanced settings of the input screen.
Is the Rat hour judged by clock time?
No. It is judged by local mean time. Seoul runs about 32 minutes behind standard time, so 11:20 p.m. on the clock is 10:48 p.m. in local mean time, still the Hae hour (9 to 11 p.m.). In periods with daylight saving time, like 1987–1988, another hour must be subtracted on top of that. So a birth 'around 11 p.m.' is the most sensitive case, where the Day Pillar changes depending on the correction, and Sajunyang applies these corrections automatically before judging whether the birth falls in the Rat hour.
FAQ
- What if the two methods give different results?
- The practical approach is to read both Day Pillar interpretations and refer to the one that fits your life so far. On Sajunyang, change the advanced setting and enter your details twice.
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This article is reference material summarizing traditional Saju theory and does not replace medical, legal or investment judgment. Our calculation standards are published at About Sajunyang.