Why your hour pillar differs between saju apps
When two saju calendars give different hour pillars for the same birth time, the cause is almost always one of three settings: (1) longitude correction, usually −30 minutes for Korea; (2) historical daylight saving time and time-zone changes; (3) whether births between 11 pm and midnight use the late-night rat hour (yajasi). Near an hour boundary, one setting can change the hour pillar and sometimes the day pillar.
1. Longitude correction: why subtract 30 minutes
Korean Standard Time is based on 135°E, but Seoul sits near 127°E. Each degree is 4 minutes, so local solar time in Seoul runs roughly 30 minutes behind the clock. Many calendars subtract 30 minutes (or a value computed from the birthplace longitude) before assigning the hour branch.
With the correction, the rat hour runs about 23:30–01:30 and the ox hour 01:30–03:30. Without it, the rat hour is 23:00–01:00 — so a 01:10 birth is rat hour in one app and ox hour in another.
2. Daylight saving time and historical time zones
South Korea observed daylight saving time in the summers of 1948–1951, 1955–1960 and 1987–1988. Birth times recorded then are one hour ahead of standard time and need an hour subtracted. From 1954 to 1961 the standard meridian was 127.5°E (UTC+8:30), which changes the longitude correction too.
Apps that apply these corrections automatically will differ from apps that don't for anyone born in those years.
3. Late-night rat hour (yajasi)
The rat hour starts at 11 pm and crosses midnight. With the late-night rat hour applied, a birth between 11 pm and midnight keeps that day's day pillar and only the hour pillar moves to the next day's rat hour. Without it, the day pillar changes at 11 pm.
Side-by-side example
Example: Seoul, July 1987 (DST period), recorded time 00:20
| Basis | Time used | Hour branch |
|---|---|---|
| No correction | 00:20 | Rat |
| Longitude only (−30 min) | 23:50 (previous day) | Rat — day pillar depends on yajasi setting |
| DST (−60) + longitude (−30) | 22:50 (previous day) | Pig |
How to check your own hour pillar
- Check whether the birth time is within about 30 minutes of an hour boundary.
- Check whether the birth year falls in a DST or historical time-zone period.
- Check how your calendar handles longitude correction and the late-night rat hour.
- Compare again with matching settings — most differences disappear.
In Myeongsik Note
Myeongsik Note applies longitude correction (default −30 min), Korean DST and historical time zones automatically, and lets you choose the late-night rat hour in settings. The basis is printed above every chart, so you can see exactly why an hour pillar came out the way it did.
Note
Saju (Four Pillars) is traditional calendar culture. This guide explains how charts are calculated, for reference and study only. It does not predict or guarantee anything, and conventions differ between schools and calendars.
A saju calendar that shows its basis
Get Myeongsik Note on Google Play