Tomomi Imura

Tomomi Imura

An Open Web advocate and front-end engineer, who loves everything mobile, and writes about HTML5, CSS, JS, UX, tech events, gadgets, etc. She unintentionally got 15min of fame by creating The HTTP Status Cats. Also, the opinions expressed here are solely her own and do not express the views or opinions of my employer.

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Reiwa - JavaScript International Date Format and Japan's New Imperial Era

As Emperor Akihito of Japan is set to abdicate soon, the Japanese government announced on April 1st that the reign of the next emperor will be known as the Reiwa (令和) era.


You may wonder like, “Wait, Japan hasn’t adopted the Gregorian calendar system!?” – Well, yes Japan did over 100 years ago, however, Japan use both “Western” calendar and the unique Japanese Imperial year, which is based on the legendary foundation of Japan by Emperor Jimmu in 660 BC.

Anyway, what I want to talk about in my blog is not about Japanese history but JavaScript Intl.DateTimeFormat object that enable language and locale specific date and time formatting.

The ECMAScript Internationalization API was originally introduced in 2010, and currently in the 6th Edition, to help localize the output of dates, numbers, and currencies in Javascript. And this has been well supported by major browsers now. (See Can I use)

One of the property of the Intl object is DateTimeFormat, which enable language and locale specific date and time formatting.

So yes, with the Intl.DateTimeFormat object, you can automatically print out the Japan local date and time with Japanese imperial era!

Using DateTimeFormat

Without specifying a locale / language, DateTimeFormat uses the default locale and the default time zone, so when I am on my machine in the US:

new Intl.DateTimeFormat().format(

returns today’s date in en-US as:


Specifying Locales

You can get a localized date and time by using a locale identifier (language code and a country/region code) as the locales argument, for example:

new Intl.DateTimeFormat('ru-RU').format(

// "02.04.2019"

and Japan is:

new Intl.DateTimeFormat('ja-JP').format(

// "2019/4/2"

Okay, but you still see the Gregorian calendar year here. So how can you make it fully localized with Imperial year?

Well, the locale identifier, ja-JP, is not enough and you need to add an extention -u-ca-japanese. I don’t know exactly what it means but all I can say is ja-JP-u-ca-japanese is an awkwardly complicated local code.

Edited: Thank you, ジャンクリストフ for letting me know that -u in the identifier indicates an extension, -ca says defines a type of calendar, in this case, -japanese calendar. For more info on locale data, see: Unicode Technical Standard #35

new Intl.DateTimeFormat('ja-JP-u-ca-japanese').format(

// "31/4/2"

Now, we know it is the year 31. But what year is it? How can we know the Imperial name!?

To get the whole deal, use the options argument, in this case use era:

new Intl.DateTimeFormat('ja-JP-u-ca-japanese', {era:'long'}).format(

// ta-da! "平成31年4月2日"

Oooh yeah, now you get “平成31年4月2日”!

Let’s try one more thing with Thai locale:

new Intl.DateTimeFormat('th-TH-u-nu-thai', {era:'long'}).format(

// "๒ ๔ พุทธศักราช ๒๕๖๒"

Woooh, I have no idea how to read this, but it is the year 2562 in Buddhist calendar!

Well, you can find details on the options and learn more about this topic on MDN Web Docs

Also, I quickly wrote up this silly little web app called Is it Reiwa (令和) yet? on This should say No until May 1, 2019, when Crown Prince Naruhito becomes a new emperor, the imperial year should be changed to 令和元年, if the new era is implemented in your browser!!! So let’s update your browser and see!

By the way, I totally didn’t blog on my own website last year, as writing technical tutorials has become a part of my day job, I don’t write when I am not paid as often. I hope I can start writing random technical posts again.

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