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Born at 11 p.m.?
— The Late and Early Ja Hour
If you were born between 11:00 and 11:59 p.m.,
different apps may give you different day pillars.
It's not that one is wrong — they just disagree on where the day begins
Divide the twenty-four hours of a day among the twelve Earthly Branches and each slot is two hours. The trouble is the first slot, the Ja hour. It starts at 11 p.m. and ends at 1 a.m. It straddles midnight.
The other eleven slots sit neatly within a single day, but the Ja hour alone spans two. Which day should the Saju of someone born in this hour belong to? That's where opinions split.
In this article 8 topics
Late and early Ja hour
These are the names for the two halves of the Ja hour, split at midnight.
So what actually matters is the one hour of the late Ja hour. That's 1/24 of a day, roughly 4.2% of births — about one person in twenty-four.
How do the two theories differ?
| Category | Ja-hour-start theory (子時日數說) | Late-Ja-hour theory (夜子時說) |
|---|---|---|
| Start of the day | 11 p.m. | Midnight (0:00) |
| Day pillar for the late Ja hour | Moves to the next day | Stays on the same day |
| Hour branch for the late Ja hour | Ja (子) | Ja (子) |
| Heavenly stem of the hour pillar | Based on the next day's Day Master | Based on the next day's Day Master |
| How widely used | More common in Korean manseryeok | Used by some schools |
Notice that the hour pillar's stem is the same under both. The hour's stem is derived from that day's Day Master, and since the Ja hour is the first hour of the day, both theories use the next day's Day Master. So under the late-Ja-hour theory you get a somewhat unusual shape: the day pillar is that day's, but only the hour pillar's stem is based on the next day. This awkwardness is the main argument of those who oppose the late-Ja-hour theory.
How much difference does it actually make?
Let's calculate for someone born at 11:30 p.m. on May 5, 2000.
| Category | Ja hour starts the new day | Late Ja hour (same day until midnight) |
|---|---|---|
| Day pillar | Gapja (甲子) | Gyehae (癸亥) |
| Day Master (the character that means you) | Gap (甲) — Wood, yang stem | Gye (癸) — Water, yin stem |
| Nayin element | Gold in the Sea | Great Sea Water |
| Gongmang | Sul · Hae | Ja · Chuk |
Not only are both characters of the day pillar different, but even the Day Master, the character that stands for you, changes. It goes from a yang stem to a yin stem, from Wood to Water, so the whole reading heads in a different direction. Why the day pillar matters so much is covered in Day pillars and Day Masters.
The year and month pillars don't change. Even if the date moves forward a day, the year and month pillars are set by solar terms, so they usually stay the same. However, if that day happens to fall on a solar-term boundary, they can shift too. It's rare, but it happens.
Which one is right?
There's no settled answer. This is interpretation, not fact. Whether daylight saving time was in effect is a fact you can check, but when the day begins is a matter of convention.
In Korean manseryeok today, the Ja-hour-start theory is more widely used. That said, the Mutal Unnie app's default is the late-Ja-hour theory (births in the 11 p.m. hour keep that day's day pillar), which is compatible with standard manseryeok. However, for late-Ja-hour births, it shows both results side by side. We think hiding one and pretending the other is the right answer would be worse.
What to check before that
This part is important. Whether a birth falls in the late Ja hour must be judged by the time after corrections.
Do it in a different order and you'll get it wrong. For example, a record of 11:30 p.m. in summer 1988 becomes 10:30 p.m. once daylight saving time is subtracted, so it's not the late Ja hour but the Hae (亥) hour. Conversely, a record of 11:50 p.m. in 1958 becomes 11:20 p.m. after correction, so it's still the late Ja hour.
When you only vaguely remember the time
If all you were told is “around midnight”, it could be the late or the early Ja hour. In that case, one approach is to calculate all three possibilities and see which of the three resulting day pillars feels more plausible to you.
But this is only a fallback. Leaving out the hour pillar is better than entering an unknown time at random. Most interpretation is possible with Three Pillars (三柱), without the hour. We've written about this in When you don't know your birth time.
Check it yourself
Enter your date and time of birth and it tells you whether it falls in the late Ja hour — and if so, shows the results of both theories side by side.
→ Open the Manseryeok calculator
If you're on the boundary, look at both
Instead of picking one side for you, Mutal Unnie shows you exactly where they diverge.
Get the app for iPhone & AndroidFAQ
What are the late and early Ja hours?
The Ja hour (11 p.m.–1 a.m.) straddles midnight, so the half before midnight is called the late Ja hour (yajasi 夜子時) (23:00–23:59), and the half after midnight the early Ja hour (jojasi 早子時) (00:00–00:59). The early Ja hour is uncontroversial; only the late Ja hour splits the date.
If I was born at 11:30 p.m., what's my day pillar?
It depends on the theory. The Ja-hour-start theory moves you to the next day, and the late-Ja-hour theory keeps you on the same day. For 11:30 p.m. on May 5, 2000, they give the Gapja (甲子) day pillar and the Gyehae (癸亥) day pillar respectively.
Which theory is right?
There's no single right answer, because it isn't a matter of fact but an interpretation of where the day begins. In Korea, the Ja-hour-start theory is more common, but the late-Ja-hour theory is used too. It's better to look at both results.
Which day's Day Master is used to derive the heavenly stem of the hour pillar?
Both theories derive it from the next day's Day Master. The hour's stem is set by that day's Day Master, and the Ja hour is the first hour of the day. So under the late-Ja-hour theory, the day pillar and the hour pillar's stem end up based on different dates.