If you're looking for general information about the 24 hour time system, here’s a quick summary.
The 24 hour systemIn the 24 hour time system, the 24 hours of the day are numbered from 1 to 24, or more precisely, 0 through 23. The first 12 hours of the day are numbered from 1 to 12, but 1 in the afternoon is numbered 13, 2 is numbered 14, and so on, until 11 at night, which is numbered 23. For midnight, see below.
Many digital clocks and watches use the 24 hour time system. If they can’t manage the 24 hour time system, they’ll need some kind of AM/PM indication, which is usually the case in countries such as the USA, where the 12 hour time system is used on digital timepieces.
Most analog clocks and watches (dials with hands) use the 12 hour time system, but a few historical or specialist devices use a dial that shows all 24 hours. These 24 hour analog dials are the subject of this web site.
To convert between the 24 and 12 hour time systems, use this diagram:
To convert to 24 hour time, add 12 to a PM time. To convert a 24 hour time to 12 hour time, subtract 12 if it’s 13 or more, then add the correct suffix (AM if the original value was less than 13, or PM if 13 or more).
The 24 hour time system used to be called continental time (in England). Only in the US (where 5% of the world's population live) is the 24 hour time system called military time.
Thomson and Thompson, the detectives in Hergé’s Destination Moon, made the classic mistake of using the 12 hour system and then forgetting the important difference between an AM time and a PM time.
The Latin tags Ante Meridiem (before the middle of the day) and Post Meridiem (after), abbreviated to AM and PM, are required to indicate the difference between, say, 1:34 in the morning (01:34) and 1:34 in the afternoon (13:34). You don’t need to say AM and PM if you use the 24 hour time system.
Here’s a short video which compares the 12 hour and 24 hour time systems:
The Double-XII systemMany older 24 hour analog clocks and watches shown on this site don't use the 24 hour time system. They use the double-XII system, which consists of two sets of 1 to 12, often in Roman numerals, one for the night/morning, one for the afternoon/evening). More recent models usually use the 24 hour time system. For example, on an old clock, the time of, say, sunset might be displayed as VI (6), but with the hour hand pointing to the 'afternoon' VI, rather than the 'morning' VI.
MidnightA 24 hour day can have two midnights: one at the start of the day, and the other at the very end. In the 24 hour time system, the times are written as 00:00 and 24:00. This sign in Switzerland gives the general idea:
For practical reasons, though, clocks can't show both 00:00 and 24:00. Most 24 hour analog clocks have settled on showing 0 or 00 to start the day. A few show 24. I think this is because it emphasizes the type of clock you’re looking at.
In the 12 hour time system, midnight is usually written as 12 AM, and midday (noon) as 12 PM. People find this confusing because 12 AM is before 1 AM yet after 11 PM - the sequence 11 PM - 12 AM - 1 AM confuses because the suffix changes an hour before the value switches back to 1. The period of time between 11 AM and 12 AM is either 11 or 13 hours, but not 1, which you might expect. It’s also confusing that “12 AM” appears to mean 12 hours before noon (“ante meridiem”) but “1 AM” means 1 hour after starting counting from midnight, before noon.
Time standardsThe International Standard for time and date notation (ISO 8601) specifies that numeric dates should be represented in the format:
eg 2003-03-27, and that times should be represented as
where hh is the number of hours since midnight (0-24), and the seconds are optional. Time zones can also be added, but there is no provision for using the 12 hour clock.
This format is useful for computer users: lists of dates and times are easy to sort into order of occurrence. For example, you can use a numerical version of the ISO date and time (eg 20050621102746 for 2005-06-21 10:27:46) as a timestamp for databases and spreadsheets, making searching and sorting easy and predictable.
Speaking the timeThe 24 hour time system is used throughout the world whenever people want to specify the time precisely and unambiguously. Scientists, technologists, the military, and anyone involved with travel and international affairs use the 24 hour time system. Its use is always advisable in any situation where an error caused by confusing or omitting the AM/PM suffixes would be more than just inconvenient.
The 24 hour time system is used in daily life throughout Western Europe. Here in England, we use both the 24 hour and 12 hour time systems: 95% of travel timetables use the 24 hour time system, and scientists and technologists generally prefer it. But in informal situations we use the 12 hour system, usually omitting the AM and PM suffixes: “10 in the morning”, “11 at night”, and so on. The evening news (22:00) is “News at 10”. (We’re also in the habit of using imperial units such as inches, miles, and pints alongside metric units.)
The BBC World Service broadcasting from London uses the 24 hour time system for its time checks: “it’s 22 hours in London”, “23 hours GMT”, “Midnight GMT”, “1 o’clock GMT”, “2 o’clock GMT”, and so on.
In France and elsewhere in Europe, the 24 hour system is used more than the 12 hour system. These days, to say 13:00 (1PM), the French say “treize heures”, the Germans say “dreizehn uhr”. But in Spain they sometimes use an equivalent to AM/PM: “Son las cuarto menos cinco de la tarde” (it is five before four in the afternoon).
In the US, as in England, the 24 hour time system (and the metric system) is less commonly used in informal situations.
The 24 hour analog clock is rarely used in public today. You will find some of the rare occurrences on the pages of this site.
Hundred hours?The english-speaking military have long used the 24 hour clock, but adopted the unfortunate habit of combining the hours and minutes figures, and dispensing with that useful colon, to form a single 4 digit number. Thus 13:00 becomes 1300 and is spoken as “thirteen hundred hours”, and 03:00 becomes 0300 (“three hundred hours”). This is an ugly and obviously incorrect usage, because 1300 minutes is really about 21.6 hours, and 1300 hours is 54 days. (You can find clocks that show hours and ‘centi-hours’ (hundredths of an hour), but the military pronounciation is technically wrong, because it is still using 60 minute hours.)
Presumably the main justification for this is that it provides a verbal way of indicating that a time before midday is using the 24 hour time system: “3 o’clock” is ambiguous for speakers who also use the 12 hour time system when they’re off duty, “3 hours” sounds like a duration rather than a moment in time, so “three hundred hours” is intended to mean 03:00. The “hundred” is actually another way of saying “:00”. Outside the military, this usage is not common - in fact, it’s most likely to be heard these days as a humorous Monty Python style allusion to the military stereotypes of bygone days.
By the way, in English schools, children are still being taught this strange usage. The national curriculum teaches both the 12 hour and the 24 hour time system, and how to convert between the two. Unfortunately, the 24 hour analog clock has not yet been introduced into the classroom.