man Date::ISO8601 (3): the three ISO 8601 numerical calendars

SYNOPSIS

use Date::ISO8601 qw(present_y);
print present_y($y);
use Date::ISO8601
qw(month_days cjdn_to_ymd ymd_to_cjdn present_ymd);
$md = month_days(2000, 2);
($y, $m, $d) = cjdn_to_ymd(2406029);
$cjdn = ymd_to_cjdn(1875, 5, 20);
print present_ymd(2406029);
print present_ymd(1875, 5, 20);
use Date::ISO8601
qw(year_days cjdn_to_yd yd_to_cjdn present_yd);
$yd = year_days(2000);
($y, $d) = cjdn_to_yd(2406029);
$cjdn = yd_to_cjdn(1875, 140);
print present_yd(2406029);
print present_yd(1875, 140);
use Date::ISO8601
qw(year_weeks cjdn_to_ywd ywd_to_cjdn present_ywd);
$yw = year_weeks(2000);
($y, $w, $d) = cjdn_to_ywd(2406029);
$cjdn = ywd_to_cjdn(1875, 20, 4);
print present_ywd(2406029);
print present_ywd(1875, 20, 4);

DESCRIPTION

The international standard ISO 8601 ``Data elements and interchange formats - Information interchange - Representation of dates and times'' defines three distinct calendars by which days can be labelled. It also defines textual formats for the representation of dates in these calendars. This module provides functions to convert dates between these three calendars and Chronological Julian Day Numbers, which is a suitable format to do arithmetic with. It also supplies functions that describe the shape of these calendars, to assist in calendrical calculations. It also supplies functions to represent dates textually in the ISO 8601 formats. ISO 8601 also covers time of day and time periods, but this module does nothing relating to those parts of the standard; this is only about labelling days.

The first ISO 8601 calendar divides time up into years, months, and days. It corresponds exactly to the Gregorian calendar, invented by Aloysius Lilius and promulgated by Pope Gregory XIII in the late sixteenth century, with AD (CE) year numbering. This calendar is applied to all time, not just to dates after its invention nor just to years 1 and later. Thus for ancient dates it is the proleptic Gregorian calendar with astronomical year numbering.

The second ISO 8601 calendar divides time up into the same years as the first, but divides the year directly into days, with no months. The standard calls this ``ordinal dates''. Ordinal dates are commonly referred to as ``Julian dates'', a mistake apparently deriving from true Julian Day Numbers, which divide time up solely into linearly counted days.

The third ISO 8601 calendar divides time up into years, weeks, and days. The years approximate the years of the first two calendars, so they stay in step in the long term, but the boundaries differ. This week-based calendar is sometimes called ``the ISO calendar'', apparently in the belief that ISO 8601 does not define any other. It is also referred to as ``business dates'', because it is most used by certain businesses to whom the week is the most important temporal cycle.

The Chronological Julian Day Number is an integral number labelling each day, where the day extends from midnight to midnight in whatever time zone is of interest. It is a linear count of days, where each day's number is one greater than the previous day's number. It is directly related to the Julian Date system: in the time zone of the prime meridian, the CJDN equals the JD at noon. By way of epoch, the day on which the Convention of the Metre was signed, which ISO 8601 defines to be 1875-05-20 (and 1875-140 and 1875-W20-4), is CJDN 2406029.

This module places no limit on the range of dates to which it may be applied. All function arguments are permitted to be "Math::BigInt" or "Math::BigRat" objects in order to achieve arbitrary range. Native Perl integers are also permitted, as a convenience when the range of dates being handled is known to be sufficiently small.

FUNCTIONS

Numbers in this API may be native Perl integers, "Math::BigInt" objects, or integer-valued "Math::BigRat" objects. All three types are acceptable for all parameters, in any combination. In all conversion functions, the most-significant part of the result (which is the only part with unlimited range) is of the same type as the most-significant part of the input. Less-significant parts of results (which have a small range) are consistently native Perl integers.

All functions "die" if given invalid parameters.

Years

present_y(YEAR)

Puts the given year number into ISO 8601 textual presentation format. For years [0, 9999] this is simply four digits. For years outside that range it is a sign followed by at least four digits.

This is the minimum-length presentation format. If it is desired to use a form that is longer than necessary, such as to use at least five digits for all year numbers (as the Long Now Foundation does), then the right tool is "sprintf" (see ``sprintf'' in perlfunc).

This format is unconditionally conformant to all versions of ISO 8601 for years [1583, 9999]. For years [0, 1582], preceding the historical introduction of the Gregorian calendar, it is conformant only where it is mutually agreed that such dates (represented in the proleptic Gregorian calendar) are acceptable. For years outside the range [0, 9999], where the expanded format must be used, the result is only conformant to ISO 8601:2004 (earlier versions lacked these formats), and only where it is mutually agreed to use this format.

Gregorian calendar

Each year is divided into twelve months, numbered [1, 12]; month number 1 is January. Each month is divided into days, numbered sequentially from 1. The month lengths are irregular. The year numbers have unlimited range.

month_days(YEAR, MONTH): The parameters identify a month, and the function returns the number of days in that month as a native Perl integer.
cjdn_to_ymd(CJDN): This function takes a Chronological Julian Day Number and returns a list of a year, month, and day.
ymd_to_cjdn(YEAR, MONTH, DAY): This performs the reverse of the translation that "cjdn_to_ymd" does. It takes year, month, and day numbers, and returns the corresponding CJDN.
present_ymd(CJDN)
present_ymd(YEAR, MONTH, DAY): Puts the given date into ISO 8601 Gregorian textual presentation format. The `extended' format (with ``-'' separators) is used. The conformance notes for "present_y" apply to this function also.
If the date is given as a (YEAR, MONTH, DAY) triplet then these are not checked for consistency. The MONTH and DAY values are only checked to ensure that they fit into the fixed number of digits. This allows the use of this function on data other than actual Gregorian dates.

Ordinal dates

Each year is divided into days, numbered sequentially from 1. The year lengths are irregular. The years correspond exactly to those of the Gregorian calendar.

year_days(YEAR): The parameter identifies a year, and the function returns the number of days in that year as a native Perl integer.
cjdn_to_yd(CJDN): This function takes a Chronological Julian Day Number and returns a list of a year and ordinal day.
yd_to_cjdn(YEAR, DAY): This performs the reverse of the translation that "cjdn_to_yd" does. It takes year and ordinal day numbers, and returns the corresponding CJDN.
present_yd(CJDN)
present_yd(YEAR, DAY): Puts the given date into ISO 8601 ordinal textual presentation format. The `extended' format (with ``-'' separators) is used. The conformance notes for "present_y" apply to this function also.
If the date is given as a (YEAR, DAY) pair then these are not checked for consistency. The DAY value is only checked to ensure that it fits into the fixed number of digits. This allows the use of this function on data other than actual ordinal dates.

Week-based calendar

Each year is divided into weeks, numbered sequentially from 1. Each week is divided into seven days, numbered [1, 7]; day number 1 is Monday. The year lengths are irregular. The year numbers have unlimited range.

The years correspond to those of the Gregorian calendar. Each week is associated with the Gregorian year that contains its Thursday and hence contains the majority of its days.

year_weeks(YEAR): The parameter identifies a year, and the function returns the number of weeks in that year as a native Perl integer.
cjdn_to_ywd(CJDN): This function takes a Chronological Julian Day Number and returns a list of a year, week, and day.
ywd_to_cjdn(YEAR, WEEK, DAY): This performs the reverse of the translation that "cjdn_to_ywd" does. It takes year, week, and day numbers, and returns the corresponding CJDN.
present_ywd(CJDN)
present_ywd(YEAR, WEEK, DAY): Puts the given date into ISO 8601 week-based textual presentation format. The `extended' format (with ``-'' separators) is used. The conformance notes for "present_y" apply to this function also.
If the date is given as a (YEAR, WEEK, DAY) triplet then these are not checked for consistency. The WEEK and DAY values are only checked to ensure that they fit into the fixed number of digits. This allows the use of this function on data other than actual week-based dates.

AUTHOR

Andrew Main (Zefram) <[email protected]>

COPYRIGHT

LICENSE

This module is free software; you can redistribute it and/or modify it under the same terms as Perl itself.