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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?

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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 8:40
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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 9:18
  • @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 9:26
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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
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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

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