NAME

Date::ISO8601 - the three ISO 8601 numerical calendars

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.

INSTALLATION

	perl Build.PL

	./Build

	./Build test

	./Build install

AUTHOR

Andrew Main (Zefram) <zefram@fysh.org>

COPYRIGHT

Copyright (C) 2006, 2007, 2009, 2011, 2017

Andrew Main (Zefram) <zefram@fysh.org>

LICENSE

This module is free software; you can redistribute it and/or modify it

under the same terms as Perl itself.