ISO-8601:2019

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Terms and definitions ​

For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

Basic concepts ​

date ​

time on the calendar time scale

NOTE

  1. Common forms of date include calendar date, ordinal date and week date.

time ​

mark attribute to an instant or a time interval on a specified time scale

NOTE

  1. The term "time" is often used in common language. However, it should only be used if the meaning is clearly visible from the context.
  2. On a time scale consisting of successive time intervals, such as a clock or calendar, distinct instants may be expressed by the same time.
  3. This definition corresponds with the definition of the term "date" in IEC 60050-113:2011, 113-01-12.

instant ​

point on the time axis

NOTE

  1. An instantaneous event occurs at a specific instant.

SOURCE: IEC 60050-113:2011, 113-01-08

time axis ​

mathematical representation of the succession in time according to the space-time model of instantaneous events along a unique axis

NOTE

  1. According to the theory of special relativity, the time axis depends on the choice of a spatial reference frame.
  2. In IEC 60050-113:2011, 113-01-03, time according to the space-time model is defined to be the one-dimensional subspace of space-time, locally orthogonal to space.

SOURCE: IEC 60050-113:2011, 113-01-07, modified

The words "according to the space-time" have been added; the phrase "special theory of relativity" has been changed to "theory of special relativity" for clarity; Note 2 to entry has been added.

time scale ​

system of ordered marks which can be attributed to instants on the time axis, one instant being chosen as the origin

NOTE

  1. A time scale may amongst others be chosen as:
    • continuous, e.g. international atomic time (TAI) (see IEC 60050-713:1998, 713-05-18);
    • continuous with discontinuities, e.g. UTC due to leap seconds, standard time due to summer time and winter time;
    • successive steps, e.g. calendars, where the time axis is split up into a succession of consecutive time intervals and the same mark is attributed to all instants of each time interval;
    • discrete, e.g. in digital techniques.

SOURCE: IEC 60050-113:2011, 113-01-11, modified

The words "amongst others" in Note 1 have been added; Notes 2 and 3 have been deleted.

time interval ​

part of the time axis limited by two instants and, unless otherwise stated, the limiting instants themselves

SOURCE: IEC 60050-113:2011, 113-01-10, modified

The words "and, unless otherwise stated, the limiting instants themselves" have been added; the Notes have been deleted.

time scale unit ​

unit of measurement of a duration

EXAMPLE

  1. Calendar year, calendar month and calendar day are time scale units of the Gregorian calendar.
  2. Clock hour, clock minutes and clock seconds are time scale units of the 24-hour clock.

duration ​

non-negative quantity of time equal to the difference between the final and initial instants of a time interval

NOTE

  1. The duration is one of the base quantities in the International System of Quantities (ISQ) on which the International System of Units (SI) is based. The term "time" instead of "duration" is often used in this context and also for an infinitesimal duration.
  2. For the term "duration", expressions such as "time" or "time interval" are often used, but the term "time" is not recommended in this sense and the term "time interval" is deprecated in this sense to avoid confusion with the concept of "time interval".
  3. The exact duration of a time scale unit depends on the time scale used. For example, the duration of a year, month, week, day, hour or minute, may depend on when they occur (in a Gregorian calendar, a calendar month can have a duration of 28, 29, 30 or 31 days; in a 24-hour clock, a clock minute can have a duration of 59, 60 or 61 seconds, etc.). Therefore, the exact duration can only be evaluated if the exact duration of each is known.
  4. This definition is closely related to Note 1 of the terminology entry "duration" in IEC 60050-113:2011, 113-01-13.

clock ​

time scale suited for intra-day time measurement

EXAMPLE

The 24-hour clock is a type of clock.

NOTE

  1. clock second, clock minute and clock hour are often time scale units included in a clock.

24-hour clock ​

clock that subdivides a calendar day into 24 clock hours

NOTE

  1. UTC forms the basis of today's 24-hour clocks and is used in this document as a type of 24-hour clock, as described in 4.2.3.

recurring time interval ​

series of consecutive time intervals of identical duration

NOTE

  1. If the duration of the time intervals is measured in calendar entities, the duration of each time interval depends on the calendar dates of its start and end.
  2. If the starting instants of time intervals are repeated according to a set of rules, the "repeat rules for recurring time intervals" in ISO 8601-2:2019, Clause 5 apply.

UTC, Coordinated Universal Time ​

time scale with the same rate as International Atomic Time (TAI), but differing from TAI only by an integral number of seconds

NOTE

  1. UTC is the time standard commonly used across the world from which local time is derived.
  2. UTC is produced by the Bureau International des Poids et Mesures (BIPM), i.e. the International Bureau of Weights and Measures.
  3. TAI is a continuous time scale produced by the BIPM based on the best realizations of the SI second. TAI is a realization of Terrestrial Time (TT) with the same rate as TT, as defined by the International Astronomical Union Resolution B1.9 (2000).

SOURCE: BIPM Recommendation CCTF 3 (2017), modified

The definition of TAI has been included as Note 3 to the entry.

UTC of day ​

time of day in UTC

standard time ​

time scale derived from UTC, by a time shift established in a given location by the competent authority

EXAMPLE

  1. Some standard times do not vary within a year, such as US Eastern Standard Time (EST), US Eastern Daylight Time (EDT), Australian Western Standard Time (AWST), China Standard Time (CST), Hong Kong Standard Time (HKT), Korea Standard Time (KST) and Japanese Standard Time (JST).
  2. Some standard times vary within a year, such as US Eastern Time (ET) and Australian Central Standard Time (ACST).

NOTE

  1. The time shift of a standard time may vary in the course of a year, such as due to daylight savings.

SOURCE: IEC 60050-113:2011, 113-01-17, modified

The original NOTE has been deleted; EXAMPLE 1 and 2 and Note 1 to entry has been added.

local time scale ​

locally-applicable time scale such as standard time or a non-UTC based time scale

time of day ​

time occurring within a calendar day

NOTE

  1. Generally, time of day relates to the duration elapsed after the beginning of the day. However, this correlation breaks when changes occur in the time scale that applies to the time of day, such as time shifts and leap seconds.
  2. This definition corresponds closely with the definition of "clock time" given in IEC 60050-113:2011, 113-01-18, except that the concepts of duration and time scale are not used in this definition.

local time of day ​

time of day in a local time scale

calendar ​

time scale that uses the time scale unit of calendar day as its basic unit

EXAMPLE

The Gregorian calendar is a type of calendar.

NOTE

  1. Calendar month and calendar year are time scale units often included in a calendar.

Gregorian calendar ​

calendar in general use that defines a calendar year that closely approximates the tropical year

NOTE

  1. In this document the term "Gregorian calendar" is used to refer to the time scale described in 4.2.1.

common year ​

calendar year in the Gregorian calendar that has 365 calendar days

leap year ​

calendar year in the Gregorian calendar that has 366 calendar days

NOTE

  1. A leap year is a calendar year whose year number is divisible by four and is not a centennial year, or a centennial year whose year number is divisible by four hundred.

centennial year ​

calendar year in the Gregorian calendar whose year number is divisible without remainder by one hundred

week calendar ​

calendar based on an unbounded series of contiguous calendar weeks that uses the time scale unit of calendar week as its basic unit to represent a calendar year, according to the rule that the first calendar week of a calendar year is the week including the first Thursday of that year, and that the last one is the week immediately preceding the first calendar week of the next calendar year

NOTE

  1. This rule is based on the principle that a week belongs to the calendar year to which the majority of its calendar days belong.
  2. In the week calendar, calendar days of the first and last calendar week of a calendar year may belong to the previous and the next calendar year respectively in the Gregorian calendar.
  3. The week calendar is described in 4.2.2.

leap second ​

intentional time step of one second to adjust UTC to ensure appropriate agreement with UT1, a time scale based on the rotation of the Earth

NOTE

  1. See also ITU-R TF.460-6.
  2. An inserted second is called a positive leap second and an omitted second is called a negative leap second. A positive leap second is inserted after 23:59:59Z and can be represented as 23:59:60Z. A negative leap second is achieved by the omission of 23:59:59Z. Insertion or omission takes place as determined by the International Earth Rotation and Reference Systems Service (IERS), normally on 30 June or 31 December, but if necessary, on 31 March or 30 September.

time shift ​

constant duration difference between times of two time scales

Time and date units ​

second ​

base unit of duration measurement in the International System of Units (SI)

NOTE

  1. Second is as defined by the CGPM (Confvérence générale des poids et mesures, General Conference on Weights and Measures) on the proposal of the CIPM (Comité international des poids et mesures, International Committee for Weights and Measures).
  2. See also ISO 80000-3.

clock second ​

minute ​

clock minute ​

hour ​

clock hour ​

calendar date ​

ordinal date ​

week date ​

day ​

calendar day ​

calendar day of week ​

calendar day of month ​

calendar day of year ​

week ​

calendar week ​

calendar week of year ​

month ​

calendar month ​

year ​

calendar year ​

decade ​

century ​

Representations and formats ​

date and time expression ​

date and time representation ​

time scale component ​

basic format ​

extended format ​

complete representation ​

representation with reduced precision ​

representation with decimal fraction ​

decimal sign ​

expanded representation ​