Doesn't "正午" mean 12:00-pm down to the last minute? Unless one needs to be extremely precise, "昼{ひる}十二{じゅうに}時{じ}" or "午前{ごぜん}十二{じゅうに}時{じ}" sounds better? Each means "12-pm" but the latter two have an implied leeway of 2 minutes (or so)?
btw, is there a word that means exactly 12:00-am (midnight)?
In terms of strictness in daily conversations, I feel 正午 and 昼12時 are the same. Whether you say 正午に会いましょう or 昼12時に会いましょう doesn't really matter. Whether you can come at 12:03 PM depends on the situation.
If you absolutely need to indicate you need minute-level precision for some reason, don't use 正午 or 12時 anyway. Say 12時[0]{れい}分[0]{れい}秒ぴったり, or whatever.
Saying 午前12時 is not recommended because it's really confusing. 午後0時 is safer: Do 午前 / 午後 change depending on minutes
Strict "midnight" is 深夜0時 or 午前0時. 真夜中【まよなか】 is a close word which can sometimes mean strictly 0:00 AM, but I feel 真夜中 tends to be less strict than "midnight" because it usually means "the middle of the night."
I think 正午 means 12:00 (military time) and only 12:00.
I found this site about 正午.
http://www.nao.ac.jp/faq/a0401.html
正午 can be said 午後0時 or 午前12時, because 12:00 (m.t.) is 1 hr past from 午前11時(11:00 m.t.) and 1 hr before 午後1時(13:00 m.t.)
When you take a look at the document made in 5th year of Meiji, it is written (= determined) that 正午 means 午前12時 (and 午前12時 only,) and that "Midnight" means 午前0時 AND 午後12時. Why they excluded 午後0時 for 正午 is a mystery.
Here's what I thought. 午前12時 might cause confusion because some tend to think that 午前12時30分 as 0:30 m.t. (and I am one of them.)* So I think it's safer to say 午後0時 as 12:00 m.t.. 昼12時 is even more safer.
* This is true because digital clock in Japan usually shows "12:xx AM" for 0:xx m.t. in am-pm mode. Maybe because the top of an analog clock is "12!"
btw, is there a word that means exactly 12:00-am (midnight)?
Yes, there are. One of them is "正子{しょうし}."
正午【しょうご】 is the noon 12-1 pm. I say that using numbers adds more precision than just saying noon. So if you want the 12 p.m. exactly, I'd use "昼【ひる】十二【じゅうに】時【じ】" or "午前【ごぜん】十二【じゅうに】時【じ】".
For midnight, you can use 真夜中【まよなか】, 正子【しょうし】, 子夜【しや】, 夜半【やはん】, or 午前【ごぜん】[0時]{れいじ}.
