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);
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.
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.
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.
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.
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.
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.