ISO 34000:2023
(Main)Date and time — Vocabulary
Date and time — Vocabulary
This document defines terms related to date and time, from fundamental concepts to those of their usage and representation.
Date et l'heure — Vocabulaire
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Standards Content (Sample)
INTERNATIONAL ISO
STANDARD 34000
First edition
2023-10
Date and time — Vocabulary
Date et l'heure — Vocabulaire
Reference number
© ISO 2023
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms related to general concepts .1
4 Terms related to time scales . 4
5 Terms related to clock systems . .7
6 Terms related to calendar systems . 8
7 Terms related to time scale units .10
8 Terms related to expressions and representations .15
Bibliography .18
Index .19
iii
Foreword
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iv
Introduction
ISO documents relating to date and time concepts have been available since 1971.
This document presents terms and definitions for selected concepts relevant to date and time concepts
and of their representation.
Specifically, the terminology presented in this document:
— serves as a sound basis in the understanding of date and time;
— guides new developments in the field by underpinning mutual understanding;
— serves as a quick and handy reference for those newly inaugurated to this field.
In this document, the decimal sign is a comma on the line, and each group of three digits are separated
by a small space from the preceding digits, counting from the decimal sign, in accordance with the ISO/
IEC Directives, Part 2.
However, Resolution 10 of the 22nd General Conference on Weights and Measures (Conférence Générale
des Poids et Mesures, CGPM) in 2003 stated that:
“The decimal marker shall be either a point on the line or a comma on the line.”
And reaffirmed the following resolution from Resolution 7 of the 9th CGPM, 1948:
“Numbers may be divided in groups of three in order to facilitate reading.”
In practice, the choice between these alternatives depends on customary use in the language concerned.
In the technical areas of date and time, it is customary for the decimal point always to be used, and that
numbers are not grouped, for all languages.
v
INTERNATIONAL STANDARD ISO 34000:2023(E)
Date and time — Vocabulary
1 Scope
This document defines terms related to date and time, from fundamental concepts to those of their
usage and representation.
2 Normative references
There are no normative references in this document.
3 Terms related to general concepts
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
date
time (3.2) on the calendar system (6.1) time scale (3.5)
Note 1 to entry: Common forms of date include calendar date (7.8), ordinal date (7.9) and week date (7.10).
3.2
time
mark attributed to an instant (3.4) or a time interval (3.6) on a specified time scale (3.5)
Note 1 to entry: The term “time” is often used in common language. However, it should only be used if the meaning
is clearly visible from the context.
Note 2 to entry: On a time scale consisting of successive time intervals, such as a clock system (5.1) or calendar
system (6.1), distinct instants may be expressed by the same time.
Note 3 to entry: This definition corresponds with the definition of the term “date” in IEC 60050-113:2011, 113-
01-12.
3.2.1
proper time
time (3.2) on a proper time scale (4.1)
[9]
Note 1 to entry: See ITU-R TF.2018-0 and the BIPM SI Brochure for additional information.
3.2.2
coordinate time
time on a coordinate time scale (4.2)
Note 1 to entry: Coordinate time is a mathematical coordinate in the four-dimensional space-time of the
coordinate system. For a given event, the coordinate time has the same value everywhere.
Note 2 to entry: Coordinate times are not measured; rather, they are computed from the proper times (3.2.1) of
clocks.
Note 3 to entry: The relation between coordinate time and proper time depends on the clock’s position and state
of motion in its gravitational environment and is derived by integration of the space-time interval.
[9]
Note 4 to entry: See ITU-R TF.2018-0 and BIPM SI Brochure for additional information.
3.3
time axis
mathematical representation of the succession in time according to the space-time reference of
instantaneous events along a unique axis
Note 1 to entry: According to the theory of special relativity, the time axis depends on the choice of a spatial
reference frame.
Note 2 to entry: In IEC 60050-113:2011, 113-01-03, time according to the space-time reference is defined to be
the one-dimensional subspace of space-time, locally orthogonal to space.
[SOURCE: IEC 60050-113:2011, 113-01-07, modified — In the definition, “time” is clarified as “time
according to the space-time reference”; in note 1 to entry, the phrase “special theory of relativity” has
been changed to “theory of special relativity” for clarity; note 2 to entry has been added.]
3.4
instant
point on the time axis (3.3)
Note 1 to entry: An instantaneous event occurs at a specific instant.
[SOURCE: IEC 60050-113:2011, 113-01-08]
3.5
time scale
timescale
system of ordered marks which can be attributed to instants (3.4) on the time axis (3.3), one instant
being chosen as the origin
EXAMPLE 1 TAI (4.9) is a continuous time scale.
EXAMPLE 2 UTC (4.7) is a time scale that is continuous but contains discontinuities. Discontinuities in UTC
arise from the mechanism of inserting leap seconds (4.8).
EXAMPLE 3 Local time (4.6) with periodic changing of offsets from UTC during the year, such as seasonal time
changes like summer time and winter time, results in a time scale that is continuous with discontinuities.
EXAMPLE 4 A calendar system (6.1) is a time scale composed of successive steps, with the time axis split up
into a succession of consecutive time intervals (3.6), where the same mark is attributed to all instants of each
time interval. For instance, all instants within a calendar day (7.13) are referred to by a reference to that calendar
day only.
EXAMPLE 5 In signal processing, the process of sampling results in a discrete time scale.
Note 1 to entry: The system of ordered marks may be of the following nature: continuous, continuous with
discontinuities, in successive steps, or discrete.
Note 2 to entry: The definition, notes to entry and EXAMPLEs are derived from IEC 60050-113:2011, 113-01-11,
“timescale”.
3.6
time interval
part of the time axis (3.3) limited by two instants (3.4)
Note 1 to entry: Unless otherwise stated, a time interval is by default a closed time interval (3.6.1), which includes
the limiting instants themselves.
[SOURCE: IEC 60050-113:2011, 113-01-10, modified – The original NOTEs have been deleted; note 1 to
entry has been added.]
3.6.1
closed time interval
[a,b]
time interval (3.6) that includes both the beginning instant (3.4) and the final instant
3.6.2
open time interval
(a,b)
time interval (3.6) that does not include either the beginning instant (3.4) or the final instant
3.6.3
right half-open time interval
contiguous time interval
[a,b)
time interval (3.6) that includes the beginning instant (3.4) but not the final instant
3.6.4
left half-open time interval
(a,b]
time interval (3.6) that includes the final instant (3.4) but not the beginning instant
3.6.5
recurring time interval
series of consecutive time intervals (3.6) of identical duration (3.7)
Note 1 to entry: If the duration of the time intervals is measured in calendar system (6.1) entities, the duration of
each time interval depends on the calendar dates (7.8) of its start and its end.
Note 2 to entry: If the starting instants (3.4) 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.
3.7
duration
non-negative quantity attributed to a time interval (3.6), the value of which is equal to the difference
between the quantitative times of the final instant (3.4) and the initial instant of the time interval
Note 1 to entry: Duration is one of the base quantities in the International System of Quantities (ISQ) on which SI
is based. The term “time” instead of “duration” is often used in this context and also for an infinitesimal duration.
Note 2 to entry: 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”.
Note 3 to entry: The exact duration of a time scale unit (7.1) depends on the time scale (3.5) used. For example,
the durations of a year, month, week, day, hour, or minute, may depend on when they occur (e.g. in a Gregorian
calendar, a calendar month (7.21) can have a duration of 28, 29, 30, or 31 days; in a 24-hour clock system (5.2), a
clock minute (7.5) can have a duration of 59, 60, or 61 seconds). Therefore, the exact duration of a time scale unit
can only be evaluated if the exact duration of each composing element is known.
Note 4 to entry: The SI unit of duration is second (7.2). Time scale units derived from the SI second (7.2) are
acceptable for use with the SI, namely, minute (7.4) (1 min = 60 s), hour (7.6) (1 h = 60 min = 3 600 s) and day (7.11)
(1 d = 24 h = 86 400 s). These time invariant units are used for the scales of a stopwatch with an additional scale
for the number of days, if applicable.
Note 5 to entry: Realizations of the SI-second-derived units on time intervals and the differences between SI-
derived units and the calendar or clock units are used to handle duration changes such as those due to leap
seconds (4.8) and discontinuities such as those caused by the periodic changing of offsets from UTC (4.7) during
the year. By equating clock day (7.12) to calendar day (7.13), this sequence can be continued by calendar day
to calendar year (7.23), hence allowing UTC and its time shifts (3.9) to be used in a continuous manner within
calendar time scales.
Note 6 to entry: This definition is closely related to NOTE 1 of “duration” in IEC 60050-113:2011, 113-01-13.
3.7.1
negative duration
duration (3.7) in the reverse direction to the proceeding time scale (3.5)
3.8
time of day
time (3.2) occurring within a calendar day (7.13)
Note 1 to entry: Generally, time of day relates to the duration (3.7) elapsed after the beginning of the day.
However, this correlation breaks when changes occur in the time scale (3.5) that applies to the time of day, such
as time shifts (3.9) and leap seconds (4.8).
Note 2 to entry: 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.
3.8.1
basis time of day
time of day (3.8) in a basis time scale (4.3)
3.8.2
UTC of day
time of day (3.8) in UTC (4.7)
3.8.3
local time of day
time of day (3.8) in a local time (4.6)
3.9
time shift
difference between the marks attributed to the same instant (3.4) between times (3.2) of two time scales
(3.5)
3.10
equation of time
difference between mean solar time and apparent solar time, which varies with time within a calendar
year (7.23)
Note 1 to entry: A wall clock, for instance, is a type of device that indicates mean solar time, while a sundial is a
type of device that indicates apparent solar time.
4 Terms related to time scales
4.1
proper time scale
time scale (3.5) produced by a continuously running primary frequency standard not compensated for
gravitational frequency shift
Note 1 to entry: An ideal clock, which exactly realizes the SI second (7.2), is a clock system (5.1) that is a proper
time scale.
Note 2 to entry: This definition is derived from ITU-R TF.2018-0.
4.2
coordinate time scale
time scale (3.5) independent of the equations of motion of material bodies and in the equations of
propagation of electromagnetic waves
EXAMPLE TCG (4.13), TT (4.12), UTC (4.7) and TAI (4.9).
Note 1 to entry: This definition is derived from ITU-R TF.2018-0.
4.3
basis time scale
time scale (3.5) established to serve as reference time by a competent authority
EXAMPLE GPS system time, Galileo system time, GLONASS system time and BeiDou system time, are
examples of basis time scales established by operators of global navigation satellite systems for internal use.
They differ from UTC (4.7) by integer hours (GLONASS), integer seconds (all other, except GLONASS) and small
fractions of microseconds (all).
Note 1 to entry: UTC is the recommended basis time scale for all civil and scientific applications.
Note 2 to entry: The local time (4.6) in a location is often defined as UTC plus a certain time shift (3.9), but not
necessarily in all.
4.4
standard time
time scale (3.5) derived from a basis time scale (4.3) with a time shift (3.9) established by a competent
authority
EXAMPLE 1 Some standard times vary within a year, such as US Eastern Time (ET) and Australian Central
Standard Time (ACST).
EXAMPLE 2 Some standard times do not vary within a year, such as US Eastern Standard Time (EST), US
Eastern Daylight Time (EDT), Central European Time (CET), Central European Summer Time (CEST), Australia
Western Standard Time (AWST), Korea Standard Time (KST), China Standard Time (CST), Hong Kong Standard
Time (HKT) and Japanese Standard Time (JST).
Note 1 to entry: The time shift of a standard time may vary in the course of a year, as decided by the competent
authority, e.g. for introducing daylight saving time.
Note 2 to entry: The local time (4.6) may switch between different standard times for administrative reasons, for
instance, a regulatory decision to adopt a different standard time.
Note 3 to entry: Many standard times use UTC (4.7) as their basis and are often associated with a geographical
location.
Note 4 to entry: This definition corresponds closely to, but is more general than, the definition of the term
“standard time” in IEC 60050-113:2011, 113-01-17.
4.5
adjusted time
time scale (3.5) derived from a basis time scale (4.3) with a time shift (3.9), established by a competent
authority that also defines a standard time (4.4)
EXAMPLE 1 Central European Summer Time (CEST) is an adjusted time in comparison with Central European
Time (CET), a standard time.
EXAMPLE 2 US Eastern Daylight Time (EDT) is an adjusted time in comparison with US Eastern Standard
Time (EST), a standard time.
4.6
local time
local time scale
time scale (3.5) applied locally of either a standard time (4.4) or adjusted time (4.5), as decided by a
competent authority
EXAMPLE Local time in some locations is subject to seasonal adjustments between standard times and
adjusted times. For instance, between Western European Time (WET) and Western European Daylight Time
(WEDT) and between US Pacific Standard Time (PST) and US Pacific Daylight Time (PDT).
4.7
UTC
Coordinated Universal Time
time scale (3.5) produced by the International Bureau of Weights and Measures (Bureau International
des Poids et Mesures, BIPM) with the same rate as TAI (4.9), but differing from TAI only by an integral
number of seconds (7.2)
Note 1 to entry: UTC is the only recommended time scale as basis time scale (4.3) and the basis of local time (4.6)
in most countries.
Note 2 to entry: Access to UTC is obtained through local real-time realizations UTC()k maintained by
laboratories contributing data to the calculation of UTC, identified by k.
Note 3 to entry: The International Earth Rotation and Reference Systems Service (IERS) decides on the insertion
of leap seconds (4.8) in UTC and thus on the integer second offset from TAI.
[10]
[SOURCE: 26th meeting of the CGPM (2018) , Resolution 2, modified – The word “but” has been
removed from the definition for clarity; notes 1 and 2 to entry have been updated; note 3 to entry has
been added.]
4.8
leap second
intentional time step of one second (7.2) to adjust UTC (4.7) to ensure approximate agreement with UT1
(4.10), a time scale (3.5) based on the rotation of the Earth
Note 1 to entry: 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 is represented as [23:59:60Z], where the
last second of that minute represented as [23:59:60Z]. A negative leap second is achieved by the omission of
[23:59:59Z], where the last second of that minute represented as [23:59:58Z]. 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.
Note 2 to entry: See also ITU-R TF.460-6.
4.9
TAI
International Atomic Time
continuous time scale (3.5) produced by the International Bureau of Weights and Measures (Bureau
International des Poids et Mesures, BIPM) based on the best realizations of the SI second (7.2)
Note 1 to entry: TAI is a realization of TT (4.12) with nominally the same rate as that of TT. In other words, TT is
a concept, while TAI is a realized time scale.
[10]
[SOURCE: 26th meeting of the CGPM (2018) , Resolution 2, modified – Note 1 to entry has been
expanded upon for clarity.]
4.10
UT1
Universal Time
time scale (3.5) defined by the International Astronomical Union (IAU) with reference to a fixed point
on the moving equator, corresponding to the Earth rotation angle (4.11)
[6]
Note 1 to entry: This definition is derived from IAU Resolution B1.8 (2000) : Definition and use of Celestial and
Terrestrial Ephemeris Origins.
Note 2 to entry: Precise values as well as predicted values of UT1 are obtained through the publication of DUT1 (
UT1U− TC) by the International Earth Rotation and Reference Systems Service (IERS).
4.11
Earth rotation angle
angle measured along the equator of the CIP between the unit vectors directed toward the CEO
(Celestial Ephemeris Origin) and the TEO (Terrestrial Ephemeris Origin)
Note 1 to entry: The Earth’s rotation is monitored by different means, such as VLBI, tracking of GNSS satellites
and Satellite-Laser-Ranging. Data are collected and evaluated by the International Earth Rotation and Reference
Systems Service (IERS).
[6]
[SOURCE: IAU Resolution B1.8 (2000) : Definition and use of Celestial and Terrestrial Ephemeris
Origin, modified – Note 1 to entry has been added.]
4.12
TT
Terrestrial Time
time scale (3.5) differing from TCG (4.13) by a constant rate ddtt/ =−1 L , where
TT TCG G
−10
L =×6,969290134 10 is a defining constant
G
Note 1 to entry: In the definition, t represents TT and t represents TCG.
TT TCG
[7]
[SOURCE: IAU Resolution B1.9 (2000) : Re-definition of Terrestrial Time TT, modified — Italicized
single-letter symbols have been used to represent TT and TCG; note 1 to entry has been added.]
4.13
TCG
Geocentric Coordinate Time
time (3.2) coordinate of the Geocentric Reference System defined by the International Astronomical
Union (IAU)
[8]
[SOURCE: IAU Resolution A4 (1991) , Recommendation III IAU Resolution A4 (1991), Recommendation
III]
5 Terms related to clock systems
5.1
clock system
clock
time scale (3.5) suited for intra-day time measurements
EXAMPLE The 24-hour clock system (5.2) is a type of clock system.
Note 1 to entry: Clock second (7.3), clock minute (7.5) and clock hour (7.7) are often time scale units (7.1) included
in a clock system.
Note 2 to entry: The document uses the term “clock system” to distinguish a clock system from a clock instance
that someone may place on their desk.
5.2
24-hour clock system
24-hour clock
24 h clock
clock system (5.1) that subdivides a calendar day (7.13) into 24 clock hours (7.7) with marks cyclically
assigned
Note 1 to entry: UTC (4.7) forms the basis of today’s 24-hour clock systems and is often used as a type of 24-hour
clock system.
Note 2 to entry: The clock day (7.12) of the 24-hour clock system is 24 hours, with each mark having a one-to-one
correspondence with marks in the 12-hour clock system (5.3).
5.3
12-hour clock system
12-hour clock
12 h clock
clock system (5.1) that divides a calendar day (7.13) into two periods that are each subdivided into
12 clock hours (7.7), with marks cyclically assigned per period
Note 1 to entry: The two periods of the 12-hour clock system are typically represented using the labels “a.m.” and
“p.m.” with historical and geographical variations.
Note 2 to entry: The clock day (7.12) of the 12-hour clock system is 24 hours, with each mark having a one-to-one
correspondence with marks in the 24-hour clock system (5.2).
6 Terms related to calendar systems
6.1
calendar system
calendar
time scale (3.5) that uses the time scale unit (7.1) of calendar day (7
...
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