ISO8601 standard.Representation of dates and times

ISO/FDIS 8601:2000(E)

3.29 year

unit of time of which the duration equals the duration of a calendar year

3.30

year, calendar

cyclic time-interval in a calendar which is required for one revolution of the earth around the sun (approximated to an integral number of calendar days)

3.31

year, centennial

áGregorian calendarñ calendar year whose year number is divisible by hundred an integral number of times

3.32

year, common

áGregorian calendarñ calendar year that has 365 days

3.33 year, leap

áGregorian calendarñ calendar year that has 366 days

4 Fundamental principles

4.1Basic concepts

For the purpose of this International Standard, three concepts are fundamental:

¾Time-point: an instant in the laps of time regarded as dimensionless. Time-points are determined by specifying their position (i.e. their 'distance' in time from the zero-point) in a time oriented reference system.

¾Time-interval: a portion of time between two time-points. These time-points are respectively labelled “start” and “end”. Time intervals may be specified by these two time-points, by one of these time-points and the temporal distance between the points or by the temporal distance between these points only.

¾Recurring time-interval: a series of consecutive time-intervals of the same duration. Recurring time-intervals may be specified by specification of one time-interval and the number of recurrences.

¾Duration: a quantity (“length”) of time. Duration is a physical unit expressed in the units of time of the International System of Units (SI), as defined in ISO 31-1.

Both precise and approximate time-points and time-intervals can be identified by means of unique and unambiguous expressions specifying the relevant dates and times of the day. This International Standard specifies a set of rules for the representation of dates, times-of-the day, time-intervals and recurring time-intervals. The degree of precision required and obtainable can be varied by including or deleting the appropriate time elements (such as seconds).

4.2Common features, uniqueness and combinations

The decreasing order of components, left-to-right, is common to the expressions for

¾points in time;

¾dates only;

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¾times only;

¾time-intervals;

¾recurring time-intervals;

¾any abbreviation of the above.

4.3Time-units and reference systems

4.3.1Time-units

Duration referred to in this International Standard shall be expressed in one or more of the following units:

¾second: a basic unit of measurement of time in the International system of units (SI), defined in ISO 31-1:1992

¾minute: a time-unit of 60 seconds

¾hour: a time-unit of 60 minutes

¾day: a time-unit of 24 hours

¾week: a time-unit of seven days.

¾month: a time-unit of 28, 29, 30 or 31 days.

¾year: a time-unit of 12 months, considered to approximate the duration required for one revolution of the earth around the sun. See also 4.3.2.1.

4.3.2Date and time reference systems

4.3.2.1The Gregorian calendar

This International Standard uses the Gregorian calendar for the identification of calendar days.

The Gregorian calendar provides a reference system consisting of a, potentially infinite, series of contiguous calendar years. Consecutive calendar years are identified by sequentially assigned year numbers. A reference point is used which assigns the year number 1875 to the calendar year in which the “Convention du mètre” was signed at Paris.

The Gregorian calendar distinguishes common years with a duration of 365 calendar days and leap years with a duration of 366 calendar days. A leap year is a year whose year number is divisible by four an integral number of times. However, centennial years are not leap years unless they are divisible by four hundred an integral number of times.

This International Standard allows the identification of calendar years by their year number for years both before and after the introduction of the Gregorian calendar. For the determination of calendar years and year numbers only the rules mentioned above are used. For the purposes of this International Standard these rules are referred to as the Gregorian calendar. The use of this calendar for dates preceding the introduction of the Gregorian calendar (i.e. before 1582) should only be done by agreement of the partners in information interchange.

NOTE 1 In the prolaptic Gregorian calendar the calendar year [0000] is a leap year.

NOTE 2 No dates shall be inserted or deleted when determining dates in the prolaptic Gregorian calendar (this may be necessary for the calculation of dates in the Julian calendar before 1582). Also note that the year numbers of years before the

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calendar year [0001] differ from the year numbers in the “BC/AD calendar system”, where the year “1 BC” is followed by the year “1 AD”.

In the Gregorian calendar each year is divided in 12 sequential calendar months as indicated in Table 1.

Table 1 — Calendar months

4.3.2.2The calendar week

In parallel with the reference system described in 4.3.2.1, there is a reference system based on an unbounded series of contiguous calendar weeks. Each calendar week has seven calendar days as indicated in Table 2.

Table 2 — Calendar days

The reference point of the week calendar assigns Saturday to 2000 January 1.

A calendar week is identified within a calendar year by the calendar week number. This is its ordinal position within the year, applying the rule that the first calendar week of a year is the one that includes the first Thursday of that year

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and that the last calendar week of a calendar year is the week immediately preceding the first calendar week of the next calendar year.

NOTE 1 These rules provide for a calendar year to have 52 or 53 calendar weeks;

NOTE 2 The first calendar week of a calendar year may include up to three days from the previous calendar year; the last calendar week of a calendar year may include up to three days from the following calendar year;

NOTE 3 The time-interval formed by the week dates of a calendar year is not the same as the time-interval formed by the calendar dates or ordinal dates for the same year. For instance:

—Sunday 1995 January 1 is the 7th day of the 52nd week of 1994, and

—Tuesday 1996 December 31 is the 2nd day of the 1st week 1997.

NOTE 4 The rule for determining the first calendar week is equivalent with the rule “the first calendar week is the week which includes January 4”.

4.3.2.3Dates

Each calendar day can be identified by a calendar date, an ordinal date, or a week date.

A calendar date identifies a calendar day by

¾its calendar year;

¾its calendar month, and

¾its ordinal number within its calendar month. An ordinal date identifies a calendar day by

¾its calendar year, and

¾its ordinal number within the calendar year. A week date identifies a calendar day by

¾the calendar year to which its calendar week belongs;

¾the calendar week number of its calendar week within that year, and

¾its ordinal number within its calendar week.

4.4Characters used in the representations

The representations specified in this International Standard use digits, alphabetic characters and special characters specified in ISO/IEC 646. The particular use of these characters is explained in 4.5 and 5.1.

NOTE 1 Where the upper case characters are not available lower case characters may be used.

NOTE 2 Encoding of characters for the interchange of dates and times is not in the scope of this International Standard.

The space character shall not be used in the representations.

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4.5Use of separators

When required, the following characters shall be used as separators:

[-] (hyphen): to separate the time elements “year” and “month”, “year” and “week”, “year” and “day”, “month” and “day”, and “week” and “day”;

[:] (colon): to separate the time elements “hour” and “minute”, and “minute” and “second”.

[/] (solidus): to separate the two components in the representation of time-intervals or recurring time-intervals.

[#] (number sign): to separate in the representation of a recurring time-interval the time-interval and the recurrence factor.

4.6Truncation

By mutual agreement of the partners in information interchange it is permitted to omit higher order components (truncation) in applications where their presence is implied. The addition of a single hyphen in place of each omitted component may be necessary, to avoid risk of misinterpretation. These leading hyphens may be omitted in the applications where there is no risk of confusing these representations with others.

Truncation should only be used in situations where the application ensures that the value of the omitted components can be inferred unequivocally by all communicating parties.

NOTE Insufficient attention to this requirement may give rise to problems like the “millennium bug”.

When truncated representations are used provisions should be made to prevent confusion of the truncated representations, with other date and time representations used by the application.

4.7Expansion

By mutual agreement of the partners in information interchange it is permitted to expand the component identifying the calendar year, which is otherwise limited to at most four digits. This enables reference to dates and times in calendar years outside the range supported by complete representations, i.e. before the start of the year [0000] or after the end of the year [9999].

When expanded representations are used, provisions should be made to prevent confusion of the expanded representations, with other date and time representations used by the application.

4.8Leading zeros

Each date and time component in a defined representation has a defined length, and leading zeros shall be used as required.

4.9Mutual agreement

Some of the representations identified in this International Standard are only allowed by mutual agreement of the partners in information interchange. Such agreement should ensure that fields in which the representation may occur is not allowed to hold other representations that cannot be unambiguously distinguished from the agreed representation.

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5 Representations

5.1Explanations

5.1.1Characters used in place of digits or signs

[Y]represents a digit used in the time element “year”;

[M] represents a digit used in the time element “month”; [D] represents a digit used in the time element “day”; [w] represents a digit used in the time element “week”;

[h]represents a digit used in the time element “hour”;

[m] represents a digit used in the time element “minute”;

[s]represents a digit used in the time element “second”;

[n]represents digit(s), constituting a positive integer or zero;

[±] represents a plus sign [+] if in combination with the following element a positive value or zero needs to be represented, or

a minus sign [–] if in combination with the following element a negative value needs to be represented. In addition the following convention applies:

When any of the characters representing a digit is underlined, it represents zero or more digits in the corresponding time element. The number of digits is to be determined by the interchanging partners.

5.1.2Characters used as designators

[P]is used as time-interval (period) designator, preceding a data element which represents a given duration of a time-interval;

[R] is used as recurring time-interval designator, preceding a data element which represents a given duration of one time-interval of a recurring time-interval or the total expression if there is no duration data element;

[T]is used as time designator to indicate:

¾the start of the representation of local time if it is necessary or desirable to designate time of the day expressions as such,

¾the start of the representation of the time of the day in combined date and time of day expressions,

¾the start of the representation of the time-units for hour, minute or second in expressions of duration;

[W] is used as week designator, preceding a data element which represents the ordinal number of a calendar week within the year;

[Z] is used as UTC designator, immediately (without space) following a data element expressing the time of the day in Coordinated Universal Time (UTC);

In representations of duration (5.5.3.1), the following characters are also used as part of the representation when required:

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[Y] [M] [W] [D] [H] [M] [S]

5.2Dates

For ease of comparison, in all the following examples of representations of dates, the date of 12 April 1985 is used as an illustration, as applicable.

5.2.1Calendar date

In expressions of calendar dates

¾day of the month (calendar day) is represented by two digits. The first day of any month is represented by [01] and subsequent days of the same month are numbered in ascending sequence;

¾month is represented by two digits. January is represented by [01], and subsequent months are numbered in ascending sequence;

¾year is generally represented by four digits; years are numbered in ascending order according to the Gregorian calendar by values in the range [0000] to [9999]. Values in the range [0000] through [1582] shall only be used by mutual agreement of the partners in information interchange.

5.2.1.1Complete representation

When the application identifies the need for an expression only of a calendar date, then the complete representation shall be a single numeric data element comprising eight digits, where [YYYY] represents a calendar year, [MM] the ordinal number of a calendar month within the calendar year, and [DD] the ordinal number of a day within the calendar month.

5.2.1.2Representations with reduced precision

If in a given application it is sufficient to express a calendar date with less precision than a complete representation as specified in 5.2.1.1, either two, four or six digits may be omitted, the omission starting from the extreme right-hand side. The resulting representation will then indicate a month, a year or a century, as set out below. When only [DD] is omitted, a separator shall be inserted between [YYYY] and [MM], but separators shall not be used in the other representations with reduced precision.

a) A specific month

Basic format: YYYY-MM EXAMPLE 1985-04

Extended format: not applicable

b) A specific year

Basic format: YYYY EXAMPLE 1985

Extended format: not applicable

c) A specific century

Basic format: YY EXAMPLE 19 Extended format: not applicable

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5.2.1.3Truncated representations

If, by agreement, truncated representations are used the basic formats shall be as specified below. In each case hyphens that indicate omitted components shall be used only as indicated or shall be omitted.

Extended format: not applicable

f)A specific day in the implied month

5.2.1.4Expanded representations

If, by agreement, expanded representations are used, the basic formats shall be as specified below. In the examples below it has been agreed to expand the time element year with two digits.

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5.2.2Ordinal date

In expressions of ordinal dates

¾day of the year is represented by three decimal digits. The first day of any year is represented by [001] and subsequent days are numbered in ascending sequence;

¾year is represented as in 5.2.1.

5.2.2.1Complete representation

When the application identifies the need for a complete representation of an ordinal date, it shall be one of the numeric expressions as follows, where [YYYY] represents a calendar year and [DDD] the ordinal number of a day within the year.

5.2.2.2Truncated representations

If, by agreement, truncated representations are used, the basic formats shall be as specified below. In each case hyphens that indicate omitted components shall be used only as indicated or shall be omitted.

5.2.2.3Expanded representations

If, by agreement, expanded representations are used the basic formats shall be as specified below. In the examples below it has been agreed to expand the time element year with two digits.

5.2.3Week date

In expressions of week dates

¾day of the week is represented by one decimal digit. Monday shall be identified as day [1] of any calendar week, and subsequent days of the same week shall be numbered in ascending sequence to Sunday (day [7]).

¾calendar week is represented by two decimal digits. The first calendar week of a year shall be identified as [01] and subsequent weeks shall be numbered in ascending sequence.

¾year is represented as in 5.2.1.

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5.2.3.1Complete representation

When the application identifies the need for a complete representation of a date identified by calendar week and day numbers, it shall be one of the alphanumeric expressions as follows, where [YYYY] represents a calendar year, [W] is the week designator, [ww] represents the ordinal number of a calendar week within the year, and [D] represents the ordinal number of a day within the calendar week.

5.2.3.2Representation with reduced precision

If the degree of precision required permits, one digit may be omitted from the representation in 5.2.3.1.

5.2.3.3Truncated representations

If, by agreement, truncated representations are used the basic formats shall be as specified below. In each case hyphens that indicate omitted components, shall be used only as indicated or shall be omitted

f)Week only of the implied year

Extended format: not applicable

NOTE 1 5.2.3.3 includes the definition of representations which are both truncated and have reduced precision.

NOTE 2 Logically the representations should be [--W-D], but the first hyphen is superfluous and, therefore, it has been omitted.