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?
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time
Do 午前 / 午後 change depending on minutes
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](https://en.wikipedia.org/wiki/12-hour_clock#Confusion_at_noon_and_midnight),
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.
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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](http://law.e-gov.go.jp/htmldata/M05/M05SE337.html) 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:

  * [午前１２時？ 午後０時？](http://jjy.nict.go.jp/QandA/FAQ/12am-or-0pm-J.html) by National Institute of Information and Communications Technology
  * [正午は午前12時？それとも、午後12時？](http://www.nao.ac.jp/faq/a0401.html) by National Astronomical Observatory of Japan
  * [真夜中と正午の時刻表現](https://ja.wikipedia.org/w/index.php?title=%E5%8D%88%E5%89%8D%E3%81%A8%E5%8D%88%E5%BE%8C&oldid=44423281#.E7.9C.9F.E5.A4.9C.E4.B8.AD.E3.81.A8.E6.AD.A3.E5.8D.88.E3.81.AE.E6.99.82.E5.88.BB.E8.A1.A8.E7.8F.BE)
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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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