ISO 8601-2:2019/Amd 1:2025

International
Standard
ISO 8601-2
First edition
Date and time — Representations
2019-02
for information interchange —
AMENDMENT 1
Part 2:
2025-01
Extensions
AMENDMENT 1: Canonical
expressions, extensions to time scale
components and date time arithmetic
Reference number
ISO 8601-2:2019/Amd.1:2025(en) © ISO 2025

ISO 8601-2:2019/Amd.1:2025(en)
© ISO 2025
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ISO 8601-2:2019/Amd.1:2025(en)
Foreword
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iii
ISO 8601-2:2019/Amd.1:2025(en)
Date and time — Representations for information
interchange —
Part 2:
Extensions
AMENDMENT 1: Canonical expressions, extensions to time scale
components and date time arithmetic

3.1.2
Add the following terminological entries after 3.1.2.13:
3.1.2.14
canonical form
date and time expression where all its time scale components are normalised (3.1.2.15)
Note 1 to entry: The canonical form of a date and time expression implies it contains minimal underflow and no
overflow.
3.1.2.15
normalise
process to ensure time scale components have values in their defined inclusive ranges
3.1.2.16
normalised duration
duration whose time scale components have values that are normalised (3.1.2.15)
Note 1 to entry: This also applies to negative durations.
3.1.2.17
overflow
state of a time scale component with a positive value when paired with a higher-order time scale component
it is unequivocally convertible with, that holds a value representing a duration exceeding one unit of the
higher-order time scale component
Note 1 to entry: A time scale component can only be in overflow when considered against a higher-order time scale
component it is unequivocally convertible with.
Note 2 to entry: The state of overflow is considered resolved if the time scale component has a value representing a
duration less than one unit of the higher-order time scale component.
EXAMPLE 1 A calendar day time scale component representing 10 calendar days is considered in overflow when
the higher-order time scale component considered is a calendar week, as the calendar day and calendar week are an
unequivocally convertible time scale component pair.
EXAMPLE 2 A calendar day time scale component representing 35 calendar days is not considered in overflow
when the higher-order time scale component considered is a calendar month, as the calendar day and calendar month
are not an unequivocally convertible time scale component pair.

ISO 8601-2:2019/Amd.1:2025(en)
3.1.2.18
underflow
state of a time scale component with a negative value when paired with a higher-order time scale component
it is unequivocally convertible with, where the result of combining the duration represented by the higher-
order time scale component and the duration represented by the lower-order time scale component is larger
than or equal to zero
Note 1 to entry: A time scale component can only be in underflow when considered against a higher-order time scale
component it is unequivocally convertible with.
Note 2 to entry: In resolving time scale component underflows, the negative value of the lower-order time scale
component can be expressed in a semantically equivalent form using a combination of the lower-order and higher-
order time scale components with values for both larger than or equal to zero.
EXAMPLE 1 A calendar month time scale component representing −8 calendar months is considered in underflow
when the higher-order time scale component considered is a calendar year with a value of 1 (which is equivalent to 12
calendar months), as the calendar month and calendar year are an unequivocally convertible time scale component pair.
EXAMPLE 2 A calendar day time scale component representing −60 calendar days is not considered in underflow
when the higher-order time scale component considered is a calendar month, as the calendar day and calendar month
are not an unequivocally convertible time scale component pair.
EXAMPLE 3 A calendar day time scale component representing −10 calendar days is considered in underflow when
the higher-order time scale component considered is a calendar week with a value of 2, but it is not considered in
underflow when the calendar week has a value of 1, since a calendar week is equivalent to 7 calendar days, which
when combined with −10 calendar days results in a duration less than zero.

14.5
Add the following subclause after 14.4:
14.5   Time scale component overflow and underflow
Time scale components described in this document and in ISO 8601-1 are each defined to accept a range
of values, some with a defined minimum or maximum value.
A date and time expression can contain time scale components with values outside the acceptable value
ranges of those components.
— A date and time expression is considered to have a normalised duration only if it does not contain
any time scale component in the state of overflow or underflow.
— A date and time expression is considered to have an “overflow” if it contains at least one time scale
component in the state of overflow.
EXAMPLE 1 The expression '1H90M' contains an overflow in the clock minute time scale component, as
the meaning of '90M' is equivalent to the expression '1H30M'.
EXAMPLE 2 The expression '1M90S' does not contain an overflow in the clock second time scale component,
as the meaning of '90S' cannot be identically expressed in an alternative combination of these two time scale
components, given that the clock minute and clock second are not an unequivocally convertible time scale
component pair.
— A date and time expression is considered to have an “underflow” if it contains at least one time scale
component in the state of underflow.

ISO 8601-2:2019/Amd.1:2025(en)
EXAMPLE 3 The expression '1H-10M' contains an underflow in the clock minute time scale component, as
the meaning of '1H-10M' (1 hour with 10 minutes before) can be identically expressed as '50M'.
EXAMPLE 4 The expression '2M-10S' does not contain an underflow in the clock second time scale
component, as the meaning of '2M-10S' cannot be identically expressed in an alternative combination of these
two time scale components, given that the clock minute and clock second are not an unequivocally convertible
time scale component pair.
An algorithm for resolving time scale component overflows within a date and time expression is provided
in D.3.1.
An algorithm for resolving time scale component underflows within a date and time expression is pro-
vided in D.3.3.
14.6
Add the following subclause after the newly added 14.5:
14.6  Time scale component conversion boundaries
Conversion of a time scale component into another time scale component is not always possible without
loss of information.
Certain pairs of time scale components are unequivocally convertible into each other as their conversions
are governed by deterministic relationships. These conversions are transitive.
Some pairs of time scale components cannot be unequivocally converted into each other, as they depend
on context-dependent duration (see D.2.2) or speculative duration (see D.2.3).
The presence of such a time scale components pair in a date and time expression marks a “time scale com-
ponent conversion boundary”, where conversions cannot occur across the pair without loss of information.
In the Gregorian calendar with the UTC 24-hour clock system, the following pairs of time scale components
are unequivocally convertible from and to each other:
— calendar century and calendar decade;
NOTE 1 In the Gregorian calendar system, a calendar century contains 10 calendar decades.

EXAMPLE 1 The date and time expression '28C' can always be converted into '280D' as the calendar
century and calendar year are an unequivocally convertible pair.
— calendar decade and calendar year;
NOTE 2 In the Gregorian calendar system, a calendar decade contains 10 calendar years.

EXAMPLE 2 The date and time expression '33D' can always be converted into '330Y' as the calendar
decade and calendar year are an unequivocally convertible pair.
— calendar year and calendar month;
NOTE 3 In the Gregorian calendar system, a calendar year contains 12 calendar months.

EXAMPLE 3 The date and time expression '20M' can always be converted into '1Y8M' as the calendar

year and calendar month are an unequivocally convertible pair.
— calendar week and calendar day;
NOTE 4 In the Gregorian calendar system, a calendar week contains 7 calendar days.

ISO 8601-2:2019/Amd.1:2025(en)
EXAMPLE 4 The date and time expression '108D' can always be converted into '15W3D' as the calendar
week and calendar day are an unequivocally convertible pair.
— calendar day and clock hour;
NOTE 5 In the Gregorian calendar system as applied with the UTC 24-hour clock system, a calendar day

contains 24 clock hours.
EXAMPLE 5 The date and time expression 'T26H' can always be converted into '1DT2H' as the calendar

day and clock hour are an unequivocally convertible pair.
— clock hour and clock minute.
NOTE 6 In the UTC 24-hour clock system, a clock hour contains 60 clock minutes.

EXAMPLE 6 The date and time expression 'T1H120M' can always be converted into 'T3H' as the clock

hour and clock minute are an unequivocally convertible pair.
The following pairs of time scale components are not unequivocally convertible from and to each other:
— calendar year and calendar week;
NOTE 7 In the Gregorian calendar system, a calendar year contains 52 or 53 calendar weeks.

EXAMPLE 7 The date and time expression '3Y55W' cannot be unequivocally converted into '4Y3W' as the
calendar year and calendar week are not an unequivocally convertible pair.
— calendar year and calendar day
NOTE 8 In the Gregorian calendar system, a calendar year contains 365 or 366 calendar days.

EXAMPLE 8 The date and time expression '1Y366D' cannot be unequivocally converted into '2Y1D' or
'2Y' as the calendar year and calendar day is a time scale component conversion boundary pair that depends
on context-dependent duration. In this example the results are ambiguous depending on the actual calendar
year this expression is evaluated in.
— calendar month and calendar day
NOTE 9 In the Gregorian calendar system, a calendar month contains 28, 29, 30 or 31 calendar days.
EXAMPLE 9 The date and time expression '3M30D' cannot be unequivocally converted into another

expression form as the calendar month and calendar day is a time scale component conversion boundary
pair that depends on context-dependent duration. In this example the results are ambiguous depending on
the actual calendar month this expression is evaluated in.
— clock minute and clock second
NOTE 10 In the UTC 24-hour clock system, a clock minute contains 59, 60 or 61 clock seconds.
EXAMPLE 10 The date and time expression '59M120S' cannot be further simplified without further

context as the clock minute and clock second is a time scale component conversion boundary pair that
depends on context-dependent duration.
The notion of context-independent conversion extends beyond the explicit pairs of time scale components
mentioned. The result of a date and time expression does not change with time scale component conversion
as long as the time scale component conversion boundaries are not crossed. Certain date and time expres-
sions syntaxes that do not depend on context-dependent durations can also be unequivocally converted.
EXAMPLE 11 The date and time expression '59O' can be unequivocally converted into '2M28D' as the 59th

ordinal day of any Gregorian year is always February 28th.

ISO 8601-2:2019/Amd.1:2025(en)
EXAMPLE 12 The date and time expression '60O' cannot be unequivocally converted into a calendar month and
calendar day combination because the 60th ordinal day of a Gregorian year can be either February 29th in a leap
year or March 1st in a regular year.
In a date and time expression, time scale component conversion boundaries can be relaxed accordingly
when context-dependent durations can be resolved to actual values.
EXAMPLE 13 The date and time expression '2023Y60O' can be unequivocally converted into '2023Y3M1D' as
the 60th ordinal day of 2023 is March 1st. However, the date time expression '2020Y60O', despite sharing the
'60O' syntax, is converted into '2020Y2M29D' as the 60th ordinal day of 2020 is February 29th.
EXAMPLE 14 The date and time expression '2016Y12M31DT60S' can be converted into '2017Y1M1DT-1S' as a

leap second was inserted into the last clock minute on 2016-12-31.
When resolving overflow and underflow conditions, the time scale component conversion boundaries cannot
be crossed, as the boundaries are used to enforce the semantic equivalence of date and time expression.
In cases where the exact duration is not necessary or does not apply, nominal duration rules can be used
to specify nominal durations for time scale components (see D.2.3) in order to relax certain time scale
component conversion boundaries. A nominal duration rule enforces a defined value for context-depend-
ent or speculative durations.
EXAMPLE 15 A possible nominal duration rule in the UTC 24-hour clock system specifies that 1 clock minute is
always equal to 60 clock seconds, ignoring the possibility of a leap second.
EXAMPLE 16 Some weather forecast models adopt nominal duration rules to limit algorithmic complexity.

A model that uses the Gregorian calendar with the UTC 24-hour clock system adopts the following nominal
duration rules: a) 1 clock minute is always equal to 60 clock seconds; b) 1 calendar month always contains 30
calendar days. In this context, the date and time expression '2016Y12M31DT60S' is unequivocally converted into
'2017Y1M1DT1S'. The UTC leap second at '2016Y12M31DT60S' is ignored due to the adopted nominal duration
rules.
EXAMPLE 17 In financial transactions and agreements, a calendar year is sometimes specified as 360 days. To
conform with terms of an agreement for the calculation of interest rate payments, the following nominal duration
rules are adopted: a) 1 calendar year contains 360 calendar days; b) 1 calendar month contains 30 calendar days.
The 31st of the month and the 29th of February are considered to be the 30th day for the calculation of interest in
the agreement.
14.7
Add the following subclause after the newly added 14.6:
14.7  Canonical form of date and time expressions
14.7.1  General
A date and time expression can be expressed in a canonical form. The canonical form of a date and time
expression enables comparison between two expressions that potentially utilize different combinations
of time scale components and values.
14.7.2  Date and time expressions with concrete context
For date and time expressions with concrete context, it is simple to derive its canonical expression.
EXAMPLE The following date and time expressions all point to the same date: '2020Y3M30D', '2020Y4M-1D',

'2020Y2M59D' and '2019Y3M396D'. The first instance is the canonical form as it contains minimal underflow and
no overflow.
14.7.3  Context-dependent expressions

ISO 8601-2:2019/Amd.1:2025(en)
The composite duration form in effect delays resolution of a date time expression until context-dependent
durations are resolved.
For date and time expressions not anchored in context, there are additional considerations in order to
derive their canonical form.
EXAMPLE 1 The expression '18Y1M' has a canonical form of exactly itself as it does not contain any overflow

or underflow.
EXAMPLE 2 The expression '3H3M3S' has a canonical form of exactly itself as it does not contain any overflow
or underflow.
EXAMPLE 3 The expression '7Y24M' has a canonical form of '9Y', as the overflow in the calendar month
component '24M' can be expressed as '2Y'.
EXAMPLE 4 The expression '1H90M' has a canonical form of '2H30M', as the overflow in the clock minute
component '90M' can be expressed as '1M30M'.
EXAMPLE 5 The expression '1H-10M' has a canonical form of '50M', as it contains an underflow in the clock
minutes time scale component '-10M', which can be resolved by applying '-10M' with the higher-order component
of '1H' into '50M'.
EXAMPLE 6 In the expression '3DT-10M', the underflow component of '-10M' can be eliminated by the transitive
conversion of time scale components from calendar day, to clock hour, to clock minutes, into the canonical form of
'2DT23H50M'.
It is possible that date and time expressions that contain time scale components with values that are
context-dependent are not fully able
...