A calendar date is a reference to a particular day represented within a calendar system. The calendar date allows the specific day to be identified. The number of days between two dates may be calculated. For example, "25 November 2024" is ten days after "15 November 2024". The date of a particular event depends on the observed time zone. For example, the air attack on Pearl Harbor that began at 7:48 a.m. Hawaiian time on 7 December 1941 took place at 3:18 a.m. Japan Standard Time, 8 December in Japan.
ISO 8601 is an international standard covering the worldwide exchange and communication of date and time-related data. It is maintained by the International Organization for Standardization (ISO) and was first published in 1988, with updates in 1991, 2000, 2004, and 2019, and an amendment in 2022. The standard provides a well-defined, unambiguous method of representing calendar dates and times in worldwide communications, especially to avoid misinterpreting numeric dates and times when such data is transferred between countries with different conventions for writing numeric dates and times.
The modern 24-hour clock is the convention of timekeeping in which the day runs from midnight to midnight and is divided into 24 hours. This is indicated by the hours passed since midnight, from 00(:00) to 23(:59), with 24(:00) as an option to indicate the end of the day. This system, as opposed to the 12-hour clock, is the most commonly used time notation in the world today, and is used by the international standard ISO 8601.
Different conventions exist around the world for date and time representation, both written and spoken.
Date and time notation in Canada combines conventions from the United Kingdom, conventions from the United States, and conventions from France, often creating confusion. The Government of Canada specifies the ISO 8601 format for all-numeric dates. It recommends writing the time using the 24-hour clock (18:05) for maximum clarity in both Canadian English and Canadian French, but also allows the 12-hour clock (6:05 p.m.) in English.
Date and time notation in the United States differs from that used in nearly all other countries. It is inherited from one historical branch of conventions from the United Kingdom. American styles of notation have also influenced customs of date notation in Canada, creating confusion in international commerce.
In Finland, the usual way of writing dates in normal text is with the months spelled out. The format varies according to the language used. In Finnish, a full stop is placed after the day to indicate an ordinal: 31. toukokuuta 2002; furthermore, the month is in the partitive case, always marked by -ta. The month can also be written first, now in genitive case : toukokuun 31. pnä 2002. In Swedish, the full stop is not used and the month is in nominative : den 31 maj 2002.
In Poland, the official system for representing dates and times follows the international ISO 8601 standard. However, in less official use, other conventions prevail, such as the day-month-year order and several Polish language abbreviations.
Date and time notation in the United Kingdom records the date using the day–month–year format. The time can be written using either the 24-hour clock (23:59) or the 12-hour clock (11:59 p.m.).
The European Committee for Standardization (CEN) and (CENELEC) adopted ISO 8601 with EN 28601, now EN ISO 8601. As a European Norm, CEN and CENELEC member states are obligated to adopt the standard as national standard without alterations as well.
ISO 8601 has been adopted as BIS IS 7900:2001.
Thailand has adopted ISO 8601 under national standard: TIS 1111:2535 in 1992. However, Thai date and time notation reflects the country’s cultural development through the years used. The formal date format is D/M/YYYY format (1/6/25), nowadays using the Buddhist Era (BE). The full date format is day-month-year format which is written in Thai. While a 24-hour system is common for official use, colloquially, a 12-hour format with terms like “morning” and “night”(Thai :ค่ำ, pronounced [Kh̀ả]) or a modified 6-hour format is used.
Date and time notation in the Philippines varies across the country in various, customary formats. Some government agencies in the Philippines have adopted time and date representation standard based on the ISO 8601, notably the Philippines driver's license and the Unified Multi-Purpose ID.
The most formal manner of expressing the full date and/or time in South Korea is to suffix each of the year, month, day, ante/post-meridiem indicator, hour, minute and second with the corresponding unit and separating each with a space:
Date and time notation in Vietnam describes methods of expressing date and time used in Vietnam.
