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English is not my native language and I am confused when writing about midnight of some day. I am mostly using 24h-format on every day basic, so I want to make sure how to use 12h-format properly.

When I am informing people they can do something between 19 April (today) 9:00 till midnight/end of the day on 22 April. How do I write it properly?

You can sign in to my diary/visit me etc. from 19th (9am) of April until 22nd of April (12pm)

Soo... Is 22 April 12pm the same as 23 April 00am? (analogical 22 April 12:30pm is the same as 23 April 00:30am?)

What is the accepted practice?

Andrew Leach
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Aerogirl
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    Twelve o'clock noon is neither am nor pm. It is "the meridiem" and hence can be neither ante-meridiem nor post-meridiem. Thus 12.00, to avoid all confusion is best written as either 12.00 noon, or 12.00 midnight. – WS2 Apr 19 '19 at 08:40
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    Also, 12:30 in the afternoon (pm) cannot be the same as 12:30 just after midnight (am). – Andrew Leach Apr 19 '19 at 09:18
  • @AndrewLeach I know that, I am asking about 12:30PM of this day and 00:30AM the next day. I understand it the same way. I always thought it's like 12PM is 24:00 (00:00) as PM to me are hours 12:00, 13:00,14:00... 23:00, 24:00. So 12:30 PM of 19.04 to me is basically 30 minutes after today (as there is PM = 24:30) – Aerogirl Apr 19 '19 at 09:26
  • No. 12:30pm is 12:30 after noon. That cannot be the same as 12:30 in the morning. The "m" of am/pm is noon, and its scope is the twelve hours between noon and midnight (before noon for am, and after noon for pm). Once you reach midnight in either direction, it resets. – Andrew Leach Apr 19 '19 at 09:33
  • Aerogirl - we can use 12-hour (1 AM, 3 PM) or 24-hour (01:00, 15:00) representation but we do not mix them. – Michael Harvey Apr 19 '19 at 09:42
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    12 PM is noon, not midnight, so 22 April 12 PM will never be the same as any time on 23 April under any circumstances. Generally speaking, the point in time denoted by (24-hour) 22 April 24:00:00 or 23 April 00:00:00 is given as 23 April 12 AM in 12-hour time. In your case, I’d probably just rephrase and say “You can sign in/visit from 9 AM on 19 April until midnight on 22 April”; it’s unlikely people will misunderstand that. Or just use 11:59 PM instead, which is completely unambiguous. – Janus Bahs Jacquet Apr 19 '19 at 10:24
  • Also, why is it necessary to use 12-hour notation at all? 24-hour notation is increasingly commonplace in English-speaking countries, and simply writing “from 19 April 09:00 until 22 April 24:00” is completely clear and unlikely not to be understood. – Janus Bahs Jacquet Apr 19 '19 at 10:25
  • Janus, the 12-hour system is mandated by the US Constitution, isn't it? – Michael Harvey Apr 19 '19 at 10:32
  • @JanusBahsJacquet Thanks! Your answer makes me almost understand ;) Sometimes my supervisors require to use 12h notation in informational notes to US clients, so as I can't argue with them I am trying to understand. Just one thing... 12PM is noon, 11:59AM is minute before noon and 11:59PM is 23:59? – Aerogirl Apr 19 '19 at 11:11
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    @Aerogirl Yes. The clock goes 11:59 PM, 12:00 AM, 12:01 AM; and 11:59 AM, 12:00 PM, 12:01 PM. – Janus Bahs Jacquet Apr 19 '19 at 11:52
  • Also related: Why is 11 am + 1 hour == 12:00 pm? – tchrist Apr 19 '19 at 13:35
  • @JanusBahsJacquet Seems unsettling that 22 April 24:00:00 and 23 April 00:00:00 denote the same instant in time, yet aren’t even the same date. – tchrist Apr 19 '19 at 13:37
  • @tchrist I suppose it’s just a result of the fact that hours (and thereby also days) are reset at 0, whereas midnight is most commonly associated with evening rather than morning (i.e., the previous day). 24:00 is just a, perhaps somewhat artificial but at least self-evident, way to explicitly denote midnight as a property of the preceding evening rather than of the following morning. – Janus Bahs Jacquet Apr 19 '19 at 13:43

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