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Next.js 15 Fullstack (App Router + Server Actions) · Lesson

Formatting Dates, Numbers, and Plurals by Locale

Use the Intl APIs to render locale-correct dates, currencies, and pluralized strings.

Formatting Dates, Numbers, and Plurals by Locale is a free Next.js 15 Fullstack (App Router + Server Actions) lesson on CoddyKit — lesson 4 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Next.js 15 Fullstack (App Router + Server Actions) learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Why Locale-Aware Formatting Matters

A date like 06/07/2025 means June 7th to an American, but July 6th to a European. A number like 1.234 is one thousand two hundred thirty-four in the US, but one point two three four in Germany.

Hard-coding format logic breaks your app for international users. Instead, the browser and Node.js both ship the Intl namespace — a standards-based set of APIs that handle locale-aware formatting with zero third-party libraries.

  • Intl.DateTimeFormat — formats dates and times
  • Intl.NumberFormat — formats numbers, currencies, and units
  • Intl.PluralRules — selects the correct plural form

In a Next.js 15 App Router project these run on the server (in Server Components and Server Actions) as well as on the client, so you get consistent output without shipping extra JavaScript.

Detecting the Active Locale in App Router

Next.js 15 with the App Router stores the active locale in the URL segment. A common convention is /en/dashboard or /de/dashboard. Your layout.tsx receives a params prop that contains it.

The snippet below shows how to read params.locale in an async Server Component and pass it down to formatting helpers.

// app/[locale]/layout.tsx
import { ReactNode } from 'react';

interface Props {
  children: ReactNode;
  params: Promise<{ locale: string }>;
}

export default async function LocaleLayout({ children, params }: Props) {
  const { locale } = await params; // e.g. 'en', 'de', 'tr'

  return (
    <html lang={locale}>
      <body>{children}</body>
    </html>
  );
}

Formatting Dates with Intl.DateTimeFormat

Intl.DateTimeFormat accepts a locale string and an options object. You can control which date/time parts to show and in what style.

  • dateStyle: 'short', 'medium', 'long', 'full'
  • timeStyle: same options
  • Individual fields: year, month, day, hour, minute, etc.

Calling .format(date) returns a locale-correct string. Because Intl is built into Node.js, this runs safely inside a Server Component with no client bundle cost.

// lib/format-date.ts
export function formatDate(
  date: Date,
  locale: string,
  options: Intl.DateTimeFormatOptions = { dateStyle: 'long' }
): string {
  return new Intl.DateTimeFormat(locale, options).format(date);
}

// Usage examples:
// formatDate(new Date(), 'en-US') => 'June 7, 2025'
// formatDate(new Date(), 'de-DE') => '7. Juni 2025'
// formatDate(new Date(), 'tr-TR') => '7 Haziran 2025'
// formatDate(new Date(), 'ja-JP') => '2025年6月7日'

Using formatDate in a Server Component

Because Server Components run on Node.js, you can call your formatDate helper directly — no useEffect, no hydration mismatch, no extra bundle weight for the client.

The locale comes from params and is passed straight into the formatter. The result is a plain string embedded in HTML.

// app/[locale]/blog/[slug]/page.tsx
import { formatDate } from '@/lib/format-date';

interface Props {
  params: Promise<{ locale: string; slug: string }>;
}

export default async function BlogPost({ params }: Props) {
  const { locale, slug } = await params;

  // In a real app you would fetch the post from a DB or CMS
  const publishedAt = new Date('2025-06-07T09:00:00Z');

  return (
    <article>
      <time dateTime={publishedAt.toISOString()}>
        {formatDate(publishedAt, locale, { dateStyle: 'full', timeStyle: 'short' })}
      </time>
      <h1>Post: {slug}</h1>
    </article>
  );
}

Formatting Numbers and Currencies

Intl.NumberFormat handles three main use-cases:

  • Plain numbers — decimal separators and grouping differ by locale (1,234.56 vs 1.234,56)
  • Currency — symbol placement, decimal precision, and even currency name differ
  • Units — kilograms, miles, bytes, etc., formatted according to locale conventions

Always pass currency together with style: 'currency'. The currency code (ISO 4217) tells the formatter which symbol to use; the locale tells it where to put it.

// lib/format-number.ts
export function formatCurrency(
  amount: number,
  currency: string,
  locale: string
): string {
  return new Intl.NumberFormat(locale, {
    style: 'currency',
    currency,
    minimumFractionDigits: 2,
    maximumFractionDigits: 2,
  }).format(amount);
}

export function formatNumber(
  value: number,
  locale: string,
  options?: Intl.NumberFormatOptions
): string {
  return new Intl.NumberFormat(locale, options).format(value);
}

// formatCurrency(1234.5, 'USD', 'en-US') => '$1,234.50'
// formatCurrency(1234.5, 'EUR', 'de-DE') => '1.234,50 €'
// formatCurrency(1234.5, 'TRY', 'tr-TR') => '₺1.234,50'

A Standalone Intl Demo (Runnable)

The snippet below is a complete, self-contained TypeScript program that demonstrates Intl.DateTimeFormat and Intl.NumberFormat across several locales. No framework or external package is needed — you can run it with ts-node or paste it into a TypeScript playground.

const locales = ['en-US', 'de-DE', 'tr-TR', 'ja-JP', 'ar-SA'];
const date = new Date('2025-06-07T14:30:00Z');
const price = 9999.99;

for (const locale of locales) {
  const formattedDate = new Intl.DateTimeFormat(locale, {
    dateStyle: 'medium',
    timeStyle: 'short',
  }).format(date);

  const formattedPrice = new Intl.NumberFormat(locale, {
    style: 'currency',
    currency: 'USD',
  }).format(price);

  console.log(`[${locale}]  date: ${formattedDate}  |  price: ${formattedPrice}`);
}

Plural Rules — the Hidden Complexity

English has two plural forms: 1 item vs 2 items. But Arabic has six, Russian has three, and Polish has four. If you build pluralization with a simple count === 1 ? 'item' : 'items' check, your app will be wrong for most of the world.

Intl.PluralRules solves this by returning the correct CLDR plural category for a given number and locale:

  • zero
  • one
  • two
  • few
  • many
  • other

You map each category to the correct translated string in your message catalog, then let PluralRules pick the right key at runtime.

Implementing a pluralize Helper

The helper below accepts a count, a locale, and a map of plural category strings. It uses Intl.PluralRules to select the right form, then replaces a {{count}} placeholder with the actual number.

This keeps plural logic out of your translation files — the files only need the per-category templates, not branching code.

// lib/pluralize.ts
type PluralForms = Partial<Record<Intl.LDMLPluralRule, string>>;

export function pluralize(
  count: number,
  locale: string,
  forms: PluralForms
): string {
  const rules = new Intl.PluralRules(locale);
  const category = rules.select(count); // 'zero'|'one'|'two'|'few'|'many'|'other'
  const template = forms[category] ?? forms['other'] ?? '';
  return template.replace('{{count}}', String(count));
}

// English
// pluralize(1, 'en', { one: '{{count}} item', other: '{{count}} items' })
// => '1 item'

// pluralize(5, 'en', { one: '{{count}} item', other: '{{count}} items' })
// => '5 items'

// Arabic (6 forms)
// pluralize(11, 'ar', { zero:'...', one:'...', two:'...', few:'...', many:'{{count}} عنصرًا', other:'...' })
// => '11 عنصرًا'

Using pluralize in a Server Component

Combine the pluralize helper with locale-aware number formatting for a complete, production-ready solution. The cart summary below shows item count and total price, both correctly formatted for any locale.

// app/[locale]/cart/page.tsx
import { pluralize } from '@/lib/pluralize';
import { formatCurrency } from '@/lib/format-number';

interface Props {
  params: Promise<{ locale: string }>;
}

// Minimal translation catalog — in real projects use next-intl or similar
const itemForms: Record<string, Record<string, string>> = {
  'en-US': { one: '{{count}} item',     other: '{{count}} items' },
  'de-DE': { one: '{{count}} Artikel',  other: '{{count}} Artikel' },
  'tr-TR': { one: '{{count}} ürün',     other: '{{count}} ürün' },
};

export default async function CartPage({ params }: Props) {
  const { locale } = await params;
  const itemCount = 3;
  const total = 129.99;
  const forms = itemForms[locale] ?? itemForms['en-US'];

  return (
    <section>
      <p>{pluralize(itemCount, locale, forms)}</p>
      <p>Total: {formatCurrency(total, 'USD', locale)}</p>
    </section>
  );
}

Relative Time Formatting with Intl.RelativeTimeFormat

Intl.RelativeTimeFormat formats durations relative to now — 3 days ago, in 2 hours — in any locale and any numeric style.

  • numeric: 'always' — always uses numbers: 1 day ago
  • numeric: 'auto' — uses natural language when possible: yesterday

The helper below computes the best unit automatically and delegates the string to the API.

// lib/format-relative.ts
export function formatRelative(date: Date, locale: string): string {
  const rtf = new Intl.RelativeTimeFormat(locale, { numeric: 'auto' });
  const diffMs = date.getTime() - Date.now();
  const diffSec = Math.round(diffMs / 1000);

  const thresholds: [number, Intl.RelativeTimeFormatUnit][] = [
    [60, 'second'],
    [3600, 'minute'],
    [86400, 'hour'],
    [2592000, 'day'],
    [31536000, 'month'],
  ];

  for (const [limit, unit] of thresholds) {
    if (Math.abs(diffSec) < limit) {
      const divisor = unit === 'second' ? 1
        : unit === 'minute' ? 60
        : unit === 'hour' ? 3600
        : unit === 'day' ? 86400
        : 2592000;
      return rtf.format(Math.round(diffSec / divisor), unit);
    }
  }

  return rtf.format(Math.round(diffSec / 31536000), 'year');
}

// formatRelative(new Date(Date.now() - 90000), 'en-US') => '2 minutes ago'
// formatRelative(new Date(Date.now() - 90000), 'de-DE') => 'vor 2 Minuten'

Creating a Reusable FormattedDate Client Component

Sometimes you need date formatting on the client — for example, when the date changes after user interaction. A thin Client Component that reads navigator.language avoids hydration mismatches by letting the browser pick its own locale.

Notice the 'use client' directive. The component itself ships no Intl polyfill because all modern browsers and Node 18+ support the full Intl spec natively.

'use client';

import { useMemo } from 'react';

interface Props {
  date: string; // ISO string — safe to serialize from Server to Client
  options?: Intl.DateTimeFormatOptions;
}

export function FormattedDate({ date, options = { dateStyle: 'medium' } }: Props) {
  // navigator.language gives the browser's locale, e.g. 'en-GB'
  const formatted = useMemo(() => {
    return new Intl.DateTimeFormat(navigator.language, options).format(
      new Date(date)
    );
  }, [date, options]);

  return <time dateTime={date}>{formatted}</time>;
}

Knowledge Check: Plural Forms and Intl.PluralRules

Based on what you learned in this lesson, answer the following question about Intl.PluralRules and locale-aware pluralization.

Recap: Locale-Aware Formatting in Next.js 15

In this lesson you learned how to use the built-in Intl APIs to format dates, numbers, currencies, relative times, and pluralized strings correctly for any locale — with no third-party libraries required.

  • Intl.DateTimeFormat — formats dates and times; use dateStyle/timeStyle for quick output or individual field options for fine control.
  • Intl.NumberFormat — handles plain numbers, currencies (style: 'currency'), and units; always pair a currency code with the locale.
  • Intl.RelativeTimeFormat — produces locale-correct relative strings like yesterday or vor 2 Minuten.
  • Intl.PluralRules — selects the CLDR plural category (zero / one / two / few / many / other) so your translation catalog can provide the right form for every language.
  • In Next.js 15 App Router, run these formatters in Server Components to avoid hydration mismatches and bundle cost; use a 'use client' wrapper with navigator.language only when client-side locale detection is needed.

These four Intl APIs cover the vast majority of i18n formatting needs without reaching for a heavy library.

Frequently asked questions

Is the “Formatting Dates, Numbers, and Plurals by Locale” lesson free?

Yes — the full text of “Formatting Dates, Numbers, and Plurals by Locale” is free to read here on the web, and the Next.js 15 Fullstack (App Router + Server Actions) course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Next.js 15 Fullstack (App Router + Server Actions) course, upgrade to CoddyKit PRO.

What will I learn in “Formatting Dates, Numbers, and Plurals by Locale”?

Use the Intl APIs to render locale-correct dates, currencies, and pluralized strings. You practise Next.js 15 Fullstack (App Router + Server Actions) with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Next.js 15 Fullstack (App Router + Server Actions)?

No prior experience is required. Next.js 15 Fullstack (App Router + Server Actions) on CoddyKit is structured for beginners through advanced learners; this is — lesson 4 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Formatting Dates, Numbers, and Plurals by Locale” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Next.js 15 Fullstack (App Router + Server Actions) lesson?

Yes. Every Next.js 15 Fullstack (App Router + Server Actions) lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

  1. Locale Detection and Subpath Routing with Middleware
  2. Server-Side Translation Loading and Message Catalogs
  3. Localized Metadata, Sitemaps, and hreflang
  4. Formatting Dates, Numbers, and Plurals by Locale
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