24-hour clock

A 24-hour clock is a way of keeping time that divides a day into 24 hours starting from midnight and ending at midnight. It is the most common way in which time is kept around the world today. The 24-hour clock is also sometimes referred to as military time.

24-hour vs 12-hour clock

In the US, the 12-hour clock is typically used when discussing the time of day, except in certain areas such as the military, navigation, tourism, computing, or in hospitals. Unlike the 24-hour clock, which spans the entire day, the 12 hour clock only spans 12 hours. The period from 12 AM to 12 PM is the first half of the day, and the period from 12 PM to 12 AM is the second half of the day. Below is a table that shows the progress of time over the course of a day in both the 24 and 12-hour clocks.

24-hour clock 12-hour clock
00:00 12:00 AM
01:00 1:00 AM
02:00 2:00 AM
03:00 3:00 AM
04:00 4:00 AM
05:00 5:00 AM
06:00 6:00 AM
07:00 7:00 AM
08:00 8:00 AM
09:00 9:00 AM
10:00 10:00 AM
11:00 11:00 AM
12:00 12:00 PM
13:00 1:00 PM
14:00 2:00 PM
15:00 3:00 PM
16:00 4:00 PM
17:00 5:00 PM
18:00 6:00 PM
19:00 7:00 PM
20:00 8:00 PM
21:00 9:00 PM
22:00 10:00 PM
23:00 11:00 PM
00:00 12:00 AM

Note that midnight in the 24-hour clock can also be written as 24:00.

The main benefit of using the 24-hour clock system is the lack of ambiguity, since the 12-hour clock requires the specification of AM and PM, and can sometimes lead to confusion if the speaker (or writer) does not specify whether a given time occurs before or after noon. None of the hours in the 24-hour clock repeat, so there is no risk of confusion.

Neither timekeeping method is perfect however, and one of the potential sources of confusion that both methods are susceptible to occurs at midnight (00:00 or 24:00 in the 24-hour click and 12:00 AM in the 12-hour clock). For example, if someone says "midnight on Sunday," regardless whether they use the 12-hour clock or 24-hour clock it is not possible to know with certainty whether midnight on Sunday refers to the time at which Sunday is ending or beginning without being told. To get around this issue, if the person meant that Sunday were just beginning, they could say 00:01 AM or simply 00:01 on Sunday; if Sunday were ending, they could say 11:59 PM or 23:59 Sunday.