Time - AM/PM vs 24 Hour Clock

Normally the time is shown as Hours:Minutes

There are 24 Hours in a Day and 60 Minutes in each Hour.

Example: 10:25 means 10 Hours and 25 Minutes

Showing the Time

There are two main ways to show the time: "24 Hour Clock" or "AM/PM":

24 Hour Clock: the time is shown as how many hours and minutes since midnight.
AM/PM (or "12 Hour Clock"): the day is split into:
  • the 12 Hours running from Midnight to Noon (the AM hours), and
  • the other 12 Hours running from Noon to Midnight (the PM hours).

Like this (24-hour above and AM/PM below it):

 
AM
PM
Ante Meridiem*
Latin for "before midday"
Post Meridiem*
Latin for "after midday"
When:
Midnight to Noon
Noon to Midnight
24 Hour Clock:
0:00 to 11:59
12:00 to 23:59


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

Converting AM/PM to 24 Hour Clock

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

Examples: 12 Midnight = 0:00, 12:35 AM = 0:35

From 1:00 AM to 12:59 PM, no change

Examples: 11:20 AM = 11:20, 12:30 PM = 12:30

From 1:00 PM to 11:59 PM, add 12 Hours

Examples: 4:45 PM = 16:45, 11:50 PM = 23:50

Converting 24 Hour Clock to AM/PM

For the first hour of the day (0:00 to 0:59), add 12 Hours, make it "AM"

Examples: 0:10 = 12:10 AM, 0:40 = 12:40 AM

From 1:00 to 11:59, just make it "AM"

Examples: 1:15 = 1:15 AM, 11:25 = 11:25 AM

From 12:00 to 12:59, just make it "PM"

Examples: 12:10 = 12:10 PM, 12:55 = 12:55 PM

From 13:00 to 23:59, subtract 12 Hours, make it "PM"

Examples: 14:55 = 2:55 PM, 23:30 = 11:30 PM

 

Comparison Chart

Here is a side-by-side comparison of the 24 Hour Clock and AM/PM:
         
Example: on the hour
Example: 10 minutes past
24 Hour Clock
AM / PM
24 Hour Clock
AM / PM
         
0:00 12 Midnight 0:10 12:10 AM
1:00 1:00 AM   1:10 1:10 AM
2:00 2:00 AM   2:10 2:10 AM
3:00 3:00 AM   3:10 3:10 AM
4:00 4:00 AM   4:10 4:10 AM
5:00 5:00 AM   5:10 5:10 AM
6:00 6:00 AM   6:10 6:10 AM
7:00 7:00 AM   7:10 7:10 AM
8:00 8:00 AM   8:10 8:10 AM
9:00 9:00 AM   9:10 9:10 AM
10:00 10:00 AM   10:10 10:10 AM
11:00 11:00 AM   11:10 11:10 AM
12:00 12 Noon   12:10 12:10 PM
13:00 1:00 PM   13:10 1:10 PM
14:00 2:00 PM   14:10 2:10 PM
15:00 3:00 PM   15:10 3:10 PM
16:00 4:00 PM   16:10 4:10 PM
17:00 5:00 PM   17:10 5:10 PM
18:00 6:00 PM   18:10 6:10 PM
19:00 7:00 PM   19:10 7:10 PM
20:00 8:00 PM   20:10 8:10 PM
21:00 9:00 PM   21:10 9:10 PM
22:00 10:00 PM   22:10 10:10 PM
23:00 11:00 PM   23:10 11:10 PM

Midnight and Noon

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

What Day is Midnight?

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

Imagine you are told your friends 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, or
  • 23:59 or 0:01 (24-Hour Clock)

which the railroads, airlines and military actually do.

So, if 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).