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午前0-12 goes from 00:00 to 12:00 in 24h time. Do minutes affect 午前 / 午後? Is 12:30 in 24h time still 午前12時30分 or does it change to 午後 thanks to the minutes going over?

  • 午前 means A.M., 午後 means P.M. So 12:30 a.m. is... – broccoli forest Apr 21 '16 at 16:57
  • No, that'd be trying to coerce it into English and Japanese is not English. 12:00 in 24h is 12p.m., but 午前12時. – Oleg V. Volkov Apr 21 '16 at 17:03
  • Aha, now I get what you mean. – broccoli forest Apr 21 '16 at 17:22
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Legal Answer: 12:00 in 24h can be only written as 午前12時, and therefore 12:30 in 24h might be 午前12時30分, because it's defined so by an ancient Japanese law (in 1872), which somehow seems to be still in effect. There is no such thing as officially-defined 午後0時. (EDIT: As @broccoriforest says, perhaps the more accurate way to describe this situation is: "Time notation between 12:00 and 13:00 has never been officially defined in the history of Japan"...)

Practical Answer: Forget the legal answer. 午前12時 itself is a confusing expression which should be avoided in most cases. Never say, for example, 午前12時30分 because it's very, very confusing. Let's not try to figure out its meaning seriously. Instead, always use 午前0時 (midnight) or 午後0時 (noon) which are unambiguous and used widely on TV, digital clocks, etc., and it I think is consistent with the common English clock system. 12:30 in 24h should be written as 午後0時30分, or 12時30分 without 午前 or 午後.

Sources:

  • BTW, 0時 is pronounced as れいじ, right? – Oleg V. Volkov Apr 21 '16 at 19:00
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    ちなみに太政官布告は「午刻ヨリ子刻迄ヲ十二時ニ分チ午後幾時ト称候」なのに時刻表に記載がないので、12:00から13:00の名称については完全にundefinedなんですよね… – broccoli forest Apr 21 '16 at 19:01
  • Usually it's れいじ, but I think I sometimes here ぜろじ, too. Technical jargon, perhaps? – naruto Apr 21 '16 at 19:02
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The most technically correct answer would be... both 午前12時30分 and 午後12時30分 are nonexistent. Because if you apply 12-hour notation rigidly, the time range only varies from 0:00 to 12:00 (whether the end is included or not is still debatable). Hence the value 12:30 a.m/p.m are simply not allowed.

That said, in daily life we could understand it by considering whether the time belongs to morning (= before noon) or afternoon. In this sense, 12:30 in 24-hour notation would be 午後12時30分, if you have to use the number 12:30.


As I see from your comment, your concern is related to the controversy about whether noon/midnight should have AM or PM. Basically it depends on definition, but at any rate, the ambiguity only involves the very moment 12:00 AM/PM. If the time should go past noon or midnight by a single tick, it'll automatically fall under either AM or PM.

  • Nah, I'm only interested in japanese 午前 / 午後 use which seems much more logical than a.m. / p.m. For example it doesn't illogicaly (IMO) jump from 11:59am to 12:00pm in a single minute and that is much simplier. Ok, I get it that it is better to use 午後0:00. But, if I mention午前12:30, will any Japanese person automatically understand it as "count 12 hours + 30 minutes from the midnight"? That's seems like natural continuation of original simple idea. – Oleg V. Volkov Apr 21 '16 at 19:22
  • Well... if you say "any", I don't deny, but not "many". Of course, even if whoever doesn't automatically understand it, enough context will make it kind of misunderstanding-free. But in the first place, if you understand the word 午前 (as well as a.m.), it means "before noon" and not "from the midnight". – broccoli forest Apr 21 '16 at 19:40
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In normal spoken japanese people often refer to 朝の10時半 or 夜の2時, or something along that vein. (or just 今朝、or 2時から) 午後 or 午前 is quite formal but commonly heard in news broadcasts.

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