Common forms of date include: calendar date (), ordinal date (), and week date ().

The term “time” is often used in common language. However, it should only be used if the meaning is clearly visible from the context.

On a time scale consisting of successive time intervals, such as a clock () or calendar (), distinct instants may be expressed by the same time.

This definition corresponds closely with the definition of the term “date” in 113-01-12.

An instantaneous event occurs at a specific instant.

According to the theory of special relativity, the time axis depends on the choice of a spatial reference frame.

In 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.

A time scale may amongst others be chosen as:

• continuous, e.g. international atomic time (TAI) (see 713-05-18);

• continuous with discontinuities, e.g. Coordinated Universal Time (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.

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.

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”.

The exact duration of a time scale unit () depends on the time scale () used. For example, the durations 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.

This definition is closely related to NOTE 1 of “duration” in 113-01-13.

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

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 its end.

If the starting instants of time intervals are repeated according to a set of rules, the “repeat rules for recurring time intervals” in 5 apply.

UTC is a basis time scale () commonly used across the world from which local time scales are derived.

UTC is produced by the Bureau International des Poids et Mesures (BIPM), i.e. the International Bureau of Weights and Measures.

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 that of TT, as defined by IAU Resolution B1.9 (2010): Re-definition of Terrestrial Time TT.

The basis time scale is not always legally UTC in all countries.

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

Many standard times use UTC as their basis and are often associated with a geographical location. A timezone may be associated with one or more standard times.

This definition corresponds closely to, but is more general than, the definition of the term “standard time” in 113-01-17.

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 ().

This definition corresponds closely with the definition of “clock time” given in 113-01-18, except that the concepts of duration () and time scale () are not used in this definition.

calendar month () and calendar year () are time scale units often included in a calendar.

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.

This rule is based on the principle that a week belongs to the calendar year in which the majority of its calendar days () belongs.

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 ().

See also .

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.

The degree of confidence in approximation depends on the application.

The unstated part can be year, year and month, month, month and day, or year and day. It is unstated because it has not (yet) been assigned (it might be assigned in the future), or because it is classified, or unknown, or for any other reason.

The selection of this term pays homage to the phrase “movable feast” used in some calendars to indicate the rule-based determination of annual events.

A profile may refer to these conformity levels to facilitate the specification of conformance to the profile.

A community may develop a profile to describe how to carry out functions or apply features specified in a standard or family of standards in a manner relevant to that community.

A profile often describes what features are to be supported and how to apply those features. In cases where multiple methods are allowed for a required feature, a profile may select a single method. In cases where a particular function allows different interpretations, a profile may select a single interpretation or provide clarification. A profile may list out features that need not be supported. It may specify several conformity levels ().

A normal sub-year grouping can be composed of time scale units with different durations.

A clock is a type of device that measures mean solar time; a sundial is a type of device that measures apparent solar time.

A single calendar date may represent different seasons depending on local customs or location, such as the difference between the northern or southern hemispheres.

The four seasons are namely spring (), summer (), autumn (), and winter ()

As defined by the CGPM (Conférence 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 of Weights and Measures).

See also .

Clock second is in common parlance often referred to as second, however in this document clock second and second have different definitions.

See also .

The duration of a minute is 60 seconds except if modified by the insertion or deletion of a leap second

Clock minute is in common parlance often referred to as minute, however in this document clock minute and minute have different definitions.

See also .

Clock hour is in common parlance often referred to as hour, however in this document clock hour and hour have different definitions.

The term “day” applies also to the duration of any time interval () which starts at a certain time of day () on a certain calendar day and ends at the same time of day on the next calendar day.

See also .

Calendar day is in common parlance often referred to as day, however in this document calendar day and day have different definitions.

The duration () of a calendar day using the 24-hour clock () is 24 hours (); except if modified by:

• the insertion or deletion of leap seconds (), by decision of the IERS; or

• the insertion or deletion of other time intervals, as may be prescribed by local authorities to alter the time scale () of local time.

The term “week” applies also to the duration of any time interval () which starts at a certain time of day () at a certain calendar day of week () and ends at the same time of day at the same calendar day of week in the next calendar week.

The term “month” applies also to the duration of any time interval () which starts at a certain time of day () at a certain calendar day () of the calendar month and ends at the same time of day at the same calendar day of the next calendar month, if it exists.

In certain applications a month is considered as a duration of 30 calendar days, but such usage is not supported by this document.

A calendar month is in common parlance often referred to as month, however in this document calendar month and month have different definitions.

In the Gregorian calendar (), a year has 365 or 366 days (). The duration is 366 days if the corresponding time interval () begins February 28 or earlier in a leap year () or March 2 or later in a year immediately preceding a leap year. If the interval begins February 29 (on a leap year), or March 1 of a year preceding a leap year, the end date has to be agreed on. Otherwise the duration is 365 days.

The term “year” applies also to the duration of any time interval () which starts at a certain time of day () at a certain calendar date () of the calendar year and ends at the same time of day at the same calendar date of the next calendar year with the exception noted in Note 1 to entry.

Decade is also used to refer to an arbitrary duration () of 10 years, however decade is not used as such in this document.

Century is also used to refer to an arbitrary duration of 100 years, however century is not used as such in this document.

A time scale component is considered of a higher order of another, if the time scale unit it represents has a strictly larger time interval () than that of that another; the latter time scale component is therefore considered to be of a lower order.

Common usage of this term often omits the leading phrase “time scale”, such as representing a “time scale component calendar year” by just “calendar year component”. This usage is deemed accepted in this document.

The representations of the decimal signs (period or comma) and their usage rules are specified in .