A time of day is written in the 24-hour notation in the form hh:mm (for example 01:23) or hh:mm:ss (for example, 01:23:45), where hh (00 to 23) is the number of full hours that have passed since midnight, mm (00 to 59) is the number of full minutes that have passed since the last full hour, and ss (00 to 59) is the number of seconds since the last full minute. In the case of a leap second, the value of ss may extend to 60. A leading zero is added for numbers under 10. This zero is optional for the hours, but very commonly used in computer applications, where many specifications require it (for example, ISO 8601). Where subsecond resolution is required, the seconds can be a decimal fraction, that is, the fractional part follows a decimal dot or comma, as in 01:23:45.678. The most commonly used separator symbol between hours, minutes and seconds is the colon, which is also the symbol used in ISO 8601. In the past, some European countries used the dot on the line as a separator, but most national standards on time notation have since then been changed to the international standard colon. In some contexts (e.g., U.S. military, some computer protocols), no separator is used (e.g., 2359) and in some jurisdictions (France and Quebec) the letter h (for "heure") is used when indicating hours and minutes only (18h45).

In the 24-hour time notation, the day begins at midnight, 00:00, and the last minute of the day begins at 23:59. Where convenient, the notation 24:00 may also be used to refer to midnight at the end of a given date—that is, 24:00 of some day is the same time as 00:00 of the following day.

The notation 24:00 mainly serves to refer to the exact end of a day in a time interval. A typical usage is giving opening hours ending at midnight, e.g. "00:00–24:00", "07:00–24:00". Similarly, some railway timetables show 00:00 as departure time and 24:00 as arrival time. Legal contracts often run from the start date at 00:00 till the end date at 24:00. It should be stressed, however, that "24:00" is a notation for the purposes of clarity and does not represent a distinct clock time.

While the 24-hour notation does unambiguously distinguish between midnight at the start (00:00) and end (24:00) of any given date, there is no such commonly accepted distinction among users of the 12-hour notation. Therefore, style guides and military communication regulations in some English-speaking countries discourage the use of 00:00 and 24:00 even in the 24-hour notation, and recommend to report times near midnight as 23:59 or 00:01 instead, to avoid misunderstandings when such times are converted into the 12-hour notation later.

Time-of-day notations beyond 24:00 (such as 24:01 or 25:59) are neither commonly used nor covered by any relevant standards. However, they have been observed occasionally in some special contexts in Japan and Hong Kong where business hours extend beyond midnight, such as broadcast-television production and with time-validation stamps on some European public transport systems, such as those used in Copenhagen, which may show, for example, 27:45 instead of 03:45.

Most digital clocks and watches show times between 00:00 to 23:59. As a notable exception, the digital clocks of at least one European manufacturer of kitchen appliances show 24:00 for midnight, rather than the standard 00:00, with an example being Bosch Siemens microwaves.

In most countries, computers by default show the time in 24-hour notation. For example, Microsoft Windows activates the 12-hour notation by default only if a computer's language and region settings are:

Usually, users can easily switch to the 24-hour notation in such locales, without affecting any of the other regional preferences.

In Canada and the United States, the term "military time" is a synonym for the 24-hour clock. In these regions, the time of day is customarily given almost exclusively using the 12-hour clock notation, which counts the hours of the day as 12, 1, ..., 11 with suffixes 'a.m.' and 'p.m.' distinguishing the two diurnal repetitions of this sequence. The 24-hour clock is commonly known there only in a few specialist areas (military, aviation, navigation, meteorology, astronomy, computing, logistics, emergency services, hospitals), where the ambiguities of the 12-hour notation are deemed too inconvenient, cumbersome, or outright dangerous, with the military's use being historically the first and most famous example. The term "military time" has no particular meaning in most other regions of the world, where the 24-hour clock has long become a common element of every-day civilian life.

In the United States military, military time is similar to the 24-hour clock notation, with the exception that the colon is omitted and the time on the hours is often spoken as its decimal value. For instance, 6:00 a.m. would become 0600, and would be spoken as "zero six hundred", "oh six hundred", or "zero six zero zero". However none of these formatting or pronunciation details is in fact exclusively military, and all are common in the technical contexts in which the 24-hour clock is generally used in English-speaking countries.