Date::Japanese::Era(3)

NAME
Date::Japanese::Era - Conversion between Japanese Era / Gregorian calendar

SYNOPSIS
  use utf8;
  use Date::Japanese::Era;

  # from Gregorian (month + day required)
  $era = Date::Japanese::Era->new(1970, 1, 1);

  # from Japanese Era
  $era = Date::Japanese::Era->new("昭和", 52); # SHOWA

  $name      = $era->name;         # 昭和 (in Unicode)
  $gengou    = $era->gengou;       # Ditto

  $year      = $era->year;         # 52
  $gregorian = $era->gregorian_year;       # 1977

  # use JIS X0301 table for conversion
  use Date::Japanese::Era 'JIS_X0301';

  # more DWIMmy
  $era = Date::Japanese::Era->new("昭和五十二年");
  $era = Date::Japanese::Era->new("昭和52年");

DESCRIPTION
Date::Japanese::Era handles conversion between Japanese Era and Gregorian calendar.

METHODS

new
      $era = Date::Japanese::Era->new($year, $month, $day);
      $era = Date::Japanese::Era->new($era_name, $year);
      $era = Date::Japanese::Era->new($era_year_string);

    Constructs new Date::Japanese::Era instance. When constructed from Gregorian date, month and day is required. You need Date::Calc to construct from Gregorian.

    Name of era can be either of Japanese / ASCII. If you pass Japanese text, they should be in Unicode.

    Errors will be thrown if you pass byte strings such as UTF-8 or EUC-JP, since Perl doesn't understand what encoding they're in. Use the utf8 pragma if you want to write them in literals.

    Exceptions are thrown when inputs are invalid, such as non-existent era name and year combination, unknwon era-name, etc.

name
      $name = $era->name;

    returns era name in Japanese in Unicode.

gengou
    alias for name().

name_ascii
      $name_ascii = $era->name_ascii;

    returns era name in US-ASCII.

year
      $year = $era->year;

    returns year as Japanese era.

gregorian_year
      $year = $era->gregorian_year;

    returns year as Gregorian.

EXAMPLES
  use utf8;
  use Date::Japanese::Era;

  # 2001 is H-13
  my $era = Date::Japanese::Era->new(2001, 8, 31);
  printf "%s-%s", uc(substr($era->name_ascii, 0, 1)), $era->year;

  # to Gregorian
  my $era = Date::Japanese::Era->new("平成", 13); # HEISEI 13
  print $era->gregorian_year;   # 2001

  ERA NAME VALIDATION AND CONVERSION
When you construct a new object from Japanese Era and year, this module does not handle if the year does not exist for the given era, such as 平成32, since the era ended in 31. This might be problematic if you want to allow the year number to exceed its end and automatically convert to the correct era i.e. 令和2.

To do this, you can use an offset-based calculation first to get the Gregorian year, and then construct a Date::Japanese::Era object from Gregorian year, month and day, such as:

  my %offset = (
      "昭和" => 1925,
      "平成" => 1988,
      "令和" => 2018,
  );

  my $name  = "平成";
  my $year  = 33;
  my $month = 4;
  my $day   = 1;

  my $gregorian_year = $offset{$name} + $year;
  my $era = Date::Japanese::Era->new( $gregorian_year, $month, $day );

  # $era is now Reiwa 3, since Heisei 33 doesn't exist.

Similarly, to validate if the given Japanese era is valid for the given date, you can compare the era after round-tripping with Gregorian year:

  sub is_valid_era {
      my( $name, $year, $month, $day ) = @_;

      my $ok;
      eval {
          my $era1 = Date::Japanese::Era->new($name, $year);
          my $era2 = Date::Japanese::Era->new($era1->gregorian_year, $month, $day);
          $ok = $era1->name eq $era2->name;
      };

      return $ok;
  }

CAVEATS

•   Currently supported era is up to 'meiji'. And before Meiji 05.12.02, gregorius calendar was not used there, but lunar calendar was. This module does not support lunar calendar, but gives warnings in such cases ("In %d they didn't use gregorius calendar").

    To use calendar ealier than that, see DateTime::Calendar::Japanese::Era, which is based on DateTime framework and is more comprehensive.

•   There should be discussion how we handle the exact day the era has changed (former one or latter one?). This module default handles the day as newer one, but you can change so that it sticks to JIS table (older one) by saying:

      use Date::Japanese::Era 'JIS_X0301';

    For example, 1912-07-30 is handled as:

      default       Taishou 1 07-30
      JIS_X0301     Meiji 45 07-30

•   If someday current era (reiwa) is changed, Date::Japanese::Era::Table should be upgraded.

AUTHOR
Tatsuhiko Miyagawa <miyagawa@bulknews.net>

COPYRIGHT
Tatsuhiko Miyagawa, 2001-

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

SEE ALSO
DateTime::Calendar::Japanese::Era, Date::Calc, Encode