feat: add support for time-zone attribute

fixes #292

Author: davhdavh

src/relative-time-element.ts

@@ -90,6 +90,14 @@ export class RelativeTimeElement extends HTMLElement implements Intl.DateTimeFor
     }
   }
 
+  get timeZone() {
+    // Prefer attribute, then closest, then document
+    const tz =
+      this.closest('[time-zone]')?.getAttribute('time-zone') ||
+      this.ownerDocument.documentElement.getAttribute('time-zone')
+    return tz || undefined
+  }
+
   #renderRoot: Node = this.shadowRoot ? this.shadowRoot : this.attachShadow ? this.attachShadow({mode: 'open'}) : this
 
   static get observedAttributes() {
@@ -113,6 +121,7 @@ export class RelativeTimeElement extends HTMLElement implements Intl.DateTimeFor
       'lang',
       'title',
       'aria-hidden',
+      'time-zone',
     ]
   }
 
@@ -129,6 +138,7 @@ export class RelativeTimeElement extends HTMLElement implements Intl.DateTimeFor
       hour: 'numeric',
       minute: '2-digit',
       timeZoneName: 'short',
+      timeZone: this.timeZone,
     }).format(date)
   }
 
@@ -198,6 +208,7 @@ export class RelativeTimeElement extends HTMLElement implements Intl.DateTimeFor
       month: this.month,
       year: this.year,
       timeZoneName: this.timeZoneName,
+      timeZone: this.timeZone,
     })
     return `${this.prefix} ${formatter.format(date)}`.trim()
   }

test/relative-time.js

@@ -2586,4 +2586,48 @@ suite('relative-time', function () {
       })
     }
   })
+
+  suite('[timeZone]', function () {
+    test('updates when the time-zone attribute is set', async () => {
+      const el = document.createElement('relative-time')
+      el.setAttribute('datetime', '2020-01-01T12:00:00.000Z')
+      el.setAttribute('time-zone', 'America/New_York')
+      el.setAttribute('format', 'datetime')
+      el.setAttribute('hour', 'numeric')
+      el.setAttribute('minute', '2-digit')
+      el.setAttribute('second', '2-digit')
+      el.setAttribute('time-zone-name', 'longGeneric')
+      await Promise.resolve()
+      assert.equal(el.shadowRoot.textContent, 'Wed, Jan 1, 2020, 7:00:00 AM Eastern Time')
+    })
+
+    test('updates when the time-zone attribute changes', async () => {
+      const el = document.createElement('relative-time')
+      el.setAttribute('datetime', '2020-01-01T12:00:00.000Z')
+      el.setAttribute('time-zone', 'America/New_York')
+      el.setAttribute('format', 'datetime')
+      el.setAttribute('hour', 'numeric')
+      el.setAttribute('minute', '2-digit')
+      el.setAttribute('second', '2-digit')
+      await Promise.resolve()
+      const initial = el.shadowRoot.textContent
+      el.setAttribute('time-zone', 'Asia/Tokyo')
+      await Promise.resolve()
+      assert.notEqual(el.shadowRoot.textContent, initial)
+      assert.equal(el.shadowRoot.textContent, "Wed, Jan 1, 2020, 9:00:00 PM")
+    })
+
+    test('ignores empty time-zone attributes', async () => {
+      const el = document.createElement('relative-time')
+      el.setAttribute('datetime', '2020-01-01T12:00:00.000Z')
+      el.setAttribute('time-zone', '')
+      el.setAttribute('format', 'datetime')
+      el.setAttribute('hour', 'numeric')
+      el.setAttribute('minute', '2-digit')
+      el.setAttribute('second', '2-digit')
+      await Promise.resolve()
+      // Should fallback to default or system time zone
+      assert.equal(el.shadowRoot.textContent, 'Wed, Jan 1, 2020, 4:00:00 PM')
+    })
+  })
 })