AM/PM (or "12 Hour Clock"): the day is split into:the 12 Hours running from Midnight to Noon (the AM hours), andthe other 12 Hours running from Noon to Midnight (the PM hours).

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*Is that spelled "Meridiem" or "Meridian"? See here.

Converting AM/PM to 24 Hour Clock

Add 12 to any hour after Noon (and subtract 12 for the first hour of the day):

For the first hour of the day (12 Midnight to 12:59 AM), subtract 12 Hours

Converting 24 Hour Clock to AM/PM

For the first hour of the day (00:00 to 00:59), add 12 Hours, make it "AM"

AM / PM 00:0012 Midnight00:1012:10 AM 01:001:00 AM01:101:10 AM 02:002:00 AM02:102:10 AM 03:003:00 AM03:103:10 AM 04:004:00 AM04:104:10 AM 05:005:00 AM05:105:10 AM 06:006:00 AM06:106:10 AM 07:007:00 AM07:107:10 AM 08:008:00 AM08:108:10 AM 09:009:00 AM09:109:10 AM 10:0010:00 AM10:1010:10 AM 11:0011:00 AM11:1011:10 AM 12:0012 Noon12:1012:10 PM 13:001:00 PM13:101:10 PM 14:002:00 PM14:102:10 PM 15:003:00 PM15:103:10 PM 16:004:00 PM16:104:10 PM 17:005:00 PM17:105:10 PM 18:006:00 PM18:106:10 PM 19:007:00 PM19:107:10 PM 20:008:00 PM20:108:10 PM 21:009:00 PM21:109:10 PM 22:0010:00 PM22:1010:10 PM 23:0011:00 PM23:1011:10 PM

Midnight and Noon

"12 AM" and "12 PM" can cause confusion, so we prefer "12 Midnight" and "12 Noon".

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What Day is Midnight?

Midnight has another problem: there is nothing to tell us "is this the beginning or ending of the day".


Imagine your friends say they are leaving for holiday at "midnight" on 12th March, what day should you arrive to say goodbye?

Do you get there on the 11th (assuming they leave at the very start of the 12th), or the 12th (assuming they leave at the end of the 12th)?

It is better to use:

11:59 PM or 12:01 AM, or23:59 or 00:01 (24-Hour Clock)

which the railroads, airlines and military actually do.

So, when you see something like "offer ends midnight October 15th" tell them to use one minute before or after so there is no confusion!

Footnote on "Meridiem" vs "Meridian"

Should "AM" be "ante meridiem" or "ante meridian" (likewise for PM)?

The official (according to an American, Australian and British dictionary I checked), and most common spelling for AM is "ante meridiem" which is a Latin phrase. I recommend that spelling!

But people sometimes use the phrase "ante meridian" (a "meridian" in this case refers to an imaginary line in the sky when the sun is at its highest point).

Adding and Subtracting Time Clocks - Digital and Analog Sun Clock World Time Zones Earth"s Orbit and Daylight