[feature] Add era support for en,ja (closes #4591)

[eternalharvest]

Description

Abstract

This PR implements new feature which is proposed in #4591.
I intended to implement this feature to format or parse Japanese date with era name, but this feature is carefully designed so that can be used in any other country.

Historically, there are more than 200 eras in Japan.
But before 6th Meiji era, our ancestors had been using Chinese lunar calendar.
And I think it is inappropriate to represent these historical dates in Gregorian solar calendar dates.
So I define only 4 eras and 2 Christian eras in Japanese locale.

Note: Japanese Meiji government had decided to use Gregorian solar calendar system since Jan 1st, Meiji 6 (Jan 1st 1873).

Currently there is only few country which is using era names, but I think this feature also can be used in Taiwan or some Buddhism country which is using Gregorian calendar.

Thanks and regards.

Related Issue

• How to convert Gregorian year to the Japanese calendar year ? #3553 How to convert Gregorian year to the Japanese calendar year ?
• To support the Taiwan year format #2586 To support the Taiwan year format
• Add BE Year (BB, BBBB) #3262 Add BE Year (BB, BBBB)
• Buddist Calendar / Thai solar Calendar support with format code BB, BBBB #874 Buddist Calendar / Thai solar Calendar support with format code BB, BBBB

Format

	Abbr	Era Name	Narrow Name	Ordinal Year	Year of Era
MOMENT	N, NN, NNN	NNNN	NNNNN	yo	y, yy, yyy, yyyy
LDML	G, GG, GGG	GGGG	GGGGG	?	y, yy, yyy, yyyy
Single Unix Specification	n/a	%EC	n/a	%EY	%Ey, %02Ey, %03Ey, %04Ey
Java	G, GG, GGG	GGGG	GGGGG	yyyy	y, yy, yyy
Excel	g	ggg, gggg	gg	n/a	e, ee
LibreOffice	G	GGG	GG	n/a	e, ee

Example

Locale (en)

> var moment = require('moment');
> 
> moment.loale('en');
> moment('1989-01-08').format('y N');
'1989 AC'
> moment([-468, 1, 1]).format('y N');
'469 BC'

Locale (ja)

> var moment = require('moment');
> 
> moment.loale('ja');
> moment('1989-01-07').format('NNNNyoM月D日');
'昭和64年1月7日'
> moment('1989-01-08').format('NNNNyoM月D日');
'平成元年1月8日'
> moment('1989-01-07').format('Nyy.MM.DD');
'S64.01.07'
> moment('1989-01-08').format('Nyy.MM.DD');
'H01.01.08'

[coveralls]

Coverage increased (+0.2%) to 94.673% when pulling 96f5059 on eternalharvest:feature/era_names into 3062ca1 on moment:develop.

[coveralls]

Coverage decreased (-0.04%) to 94.61% when pulling 753288c on eternalharvest:feature/era_names into ad49bd6 on moment:develop.

[ichernev]

@eternalharvest the code looks decent. I think it can go in in next release.

In the definition of the eras, why is BC ending on 31 Dec. Does it automatically round the day to the end? Shouldn't it be 1st Jan 0000 the end date for one era (BC) and the begining of the new era (AC).

Also its not AC and BC. But AD and BC ;-)

[eternalharvest]

@ichernev

Sorry for late reply and thank u for reviewing my PR.

In the definition of the eras, why is BC ending on 31 Dec. Does it automatically round the day to the end? Shouldn't it be 1st Jan 0000 the end date for one era (BC) and the begining of the new era (AC).

In my implementation, the date is truncated to the day value (by the method call #startOf('day')) before evaluating era name. But I'm not sure if this implementation satisfies any other locale's requirements except for japanese locale.

In japan, the beginning time (hour, minute, second) of era is not defined by the law.
It is natural to interpret the beginning time of era as 00:00:00 (JST) of first day of Japanese era.
And I think the beginning day value of era is enough to evaluate era name.

If we redesign this feature to use datetime as the beginning time of era, I think we have to consider about timezone.

Also its not AC and BC. But AD and BC ;-)

I fixed this wrong era definition.
Than u for ur advise!

[guisehn]

Any news on this one?

[eternalharvest]

@guisehn
No news about this PR.

I think this PR satisfies with Japanese locale requirements though I'm not sure about any other locale specific problem or requirements itself.

I wish this PR will be merged if there is no concern about this PR in any other locale.

[guisehn]

@eternalharvest if I may ask, what is the workaround you use in your project for now?

I'm thinking in maybe creating a plugin so that I can avoid maintaining a fork just for my project while there's no decision about the pull request, but I'm wondering if there is already an existing plugin (I couldn't find) or workaround I could use.

[yopaz-giapnh]

How is it going? I hope this feature is supported on moment asap.

[ichernev]

Merged in 4b615b9