Experimental feature: Support absolute time format

package.json

@@ -18,7 +18,7 @@
   },
   "scripts": {
     "clean": "rm -rf dist",
-    "lint": "eslint . --ext .js,.ts && tsc --noEmit",
+    "lint": "tsc --noEmit && eslint . --ext .js,.ts",
     "lint:fix": "npm run lint -- --fix",
     "prebuild": "npm run clean && npm run lint && mkdir dist",
     "bundle": "esbuild --bundle dist/index.js --keep-names --outfile=dist/bundle.js --format=esm",

src/relative-time-element.ts

@@ -217,6 +217,18 @@ export class RelativeTimeElement extends HTMLElement implements Intl.DateTimeFor
     return `${this.prefix} ${formatter.format(date)}`.trim()
   }
 
+  #getUserPreferredAbsoluteTimeFormat(date: Date): string {
+    return new Intl.DateTimeFormat(this.#lang, {
+      day: 'numeric',
+      month: 'short',
+      year: 'numeric',
+      hour: 'numeric',
+      minute: '2-digit',
+      timeZoneName: 'short',
+      timeZone: this.timeZone,
+    }).format(date)
+  }
+
   #updateRenderRootContent(content: string | null): void {
     if (this.hasAttribute('aria-hidden') && this.getAttribute('aria-hidden') === 'true') {
       const span = document.createElement('span')
@@ -228,6 +240,16 @@ export class RelativeTimeElement extends HTMLElement implements Intl.DateTimeFor
     }
   }
 
+  #shouldDisplayUserPreferredAbsoluteTime(format: ResolvedFormat): boolean {
+    // Never override duration format with absolute format.
+    if (format === 'duration') return false
+
+    return (
+      this.ownerDocument.documentElement.getAttribute('data-prefers-absolute-time') === 'true' ||
+      this.ownerDocument.body?.getAttribute('data-prefers-absolute-time') === 'true'
+    )
+  }
+
   #onRelativeTimeUpdated: ((event: RelativeTimeUpdatedEvent) => void) | null = null
   get onRelativeTimeUpdated() {
     return this.#onRelativeTimeUpdated
@@ -477,12 +499,19 @@ export class RelativeTimeElement extends HTMLElement implements Intl.DateTimeFor
     const duration = elapsedTime(date, this.precision, now)
     const format = this.#resolveFormat(duration)
     let newText = oldText
-    if (format === 'duration') {
-      newText = this.#getDurationFormat(duration)
-    } else if (format === 'relative') {
-      newText = this.#getRelativeFormat(duration)
+
+    // Experimental: Enable absolute time if users prefers it, but never for `duration` format
+    const displayUserPreferredAbsoluteTime = this.#shouldDisplayUserPreferredAbsoluteTime(format)
+    if (displayUserPreferredAbsoluteTime) {
+      newText = this.#getUserPreferredAbsoluteTimeFormat(date)
     } else {
-      newText = this.#getDateTimeFormat(date)
+      if (format === 'duration') {
+        newText = this.#getDurationFormat(duration)
+      } else if (format === 'relative') {
+        newText = this.#getRelativeFormat(duration)
+      } else {
+        newText = this.#getDateTimeFormat(date)
+      }
     }
 
     if (newText) {
@@ -496,7 +525,7 @@ export class RelativeTimeElement extends HTMLElement implements Intl.DateTimeFor
       this.dispatchEvent(new RelativeTimeUpdatedEvent(oldText, newText, oldTitle, newTitle))
     }
 
-    if (format === 'relative' || format === 'duration') {
+    if ((format === 'relative' || format === 'duration') && !displayUserPreferredAbsoluteTime) {
       dateObserver.observe(this)
     } else {
       dateObserver.unobserve(this)

test/relative-time.js

@@ -31,6 +31,7 @@ suite('relative-time', function () {
   })
 
   teardown(() => {
+    document.body.removeAttribute('data-prefers-absolute-time')
     fixture.innerHTML = ''
     if (dateNow) {
       // eslint-disable-next-line no-global-assign
@@ -1884,6 +1885,53 @@ suite('relative-time', function () {
     }
   })
 
+  suite('experimental: [data-prefers-absolute-time]', async () => {
+    test('formats with absolute time when data-prefers-absolute-time="true"', async () => {
+      document.documentElement.setAttribute('data-prefers-absolute-time', 'true')
+      const el = document.createElement('relative-time')
+      el.setAttribute('datetime', '2022-01-01T12:00:00.000Z')
+      await Promise.resolve()
+
+      assert.match(el.shadowRoot.textContent, /[A-Z][a-z]{2} \d{1,2}, \d{4}, \d{1,2}:\d{2} (AM|PM)/)
+    })
+
+    test('does not format with absolute time when format is elapsed or duration', async () => {
+      document.documentElement.setAttribute('data-prefers-absolute-time', 'true')
+      const el = document.createElement('relative-time')
+      el.setAttribute('datetime', '2022-01-01T12:00:00.000Z')
+      el.setAttribute('format', 'elapsed')
+      await Promise.resolve()
+
+      assert.notMatch(el.shadowRoot.textContent, /[A-Z][a-z]{2} \d{1,2}, \d{4}, \d{1,2}:\d{2} (AM|PM)/)
+    })
+
+    test('does not format with absolute time when data-prefers-absolute-time="false"', async () => {
+      document.documentElement.setAttribute('data-prefers-absolute-time', 'false')
+      const el = document.createElement('relative-time')
+      el.setAttribute('datetime', new Date(Date.now() - 3 * 60 * 60 * 24 * 1000).toISOString())
+      await Promise.resolve()
+
+      assert.equal(el.shadowRoot.textContent, '3 days ago')
+    })
+
+    test('does not format with absolute time when data-prefers-absolute-time attribute is not set', async () => {
+      const el = document.createElement('relative-time')
+      el.setAttribute('datetime', new Date(Date.now() - 3 * 60 * 60 * 24 * 1000).toISOString())
+      await Promise.resolve()
+
+      assert.equal(el.shadowRoot.textContent, '3 days ago')
+    })
+
+    test('supports data-prefers-absolute-time="true" on body element too', async () => {
+      document.body.setAttribute('data-prefers-absolute-time', 'true')
+      const el = document.createElement('relative-time')
+      el.setAttribute('datetime', '2022-01-01T12:00:00.000Z')
+      await Promise.resolve()
+
+      assert.match(el.shadowRoot.textContent, /[A-Z][a-z]{2} \d{1,2}, \d{4}, \d{1,2}:\d{2} (AM|PM)/)
+    })
+  })
+
   suite('[aria-hidden]', async () => {
     test('[aria-hidden="true"] applies to shadow root', async () => {
       const now = new Date().toISOString()