diff --git a/.github/workflows/publish.yml b/.github/workflows/publish.yml
index 20bebdd..26e8fd5 100644
--- a/.github/workflows/publish.yml
+++ b/.github/workflows/publish.yml
@@ -15,7 +15,7 @@ jobs:
       - uses: actions/checkout@v3
       - uses: actions/setup-node@v3
         with:
-          node-version: 18
+          node-version: 22
           registry-url: https://registry.npmjs.org/
           cache: npm
       - run: npm install -g npm@latest
@@ -29,15 +29,15 @@ jobs:
           NODE_AUTH_TOKEN: ${{secrets.npm_token}}
   publish-github:
     name: Publish to GitHub Packages
-    runs-on: ubuntu-latest 
-    permissions: 
+    runs-on: ubuntu-latest
+    permissions:
       contents: read
       packages: write
     steps:
       - uses: actions/checkout@v3
       - uses: actions/setup-node@v3
         with:
-          node-version: 18
+          node-version: 22
           registry-url: https://npm.pkg.github.com
           cache: npm
           scope: '@github'
diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 20ee438..7a23343 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -14,7 +14,7 @@ jobs:
     - name: Use Node.js 18.x
       uses: actions/setup-node@v3
       with:
-        node-version: 18.x
+        node-version: 22
     - name: npm install, build, and test
       run: |
         npm it
diff --git a/src/duration.ts b/src/duration.ts
index 74cb017..58492b2 100644
--- a/src/duration.ts
+++ b/src/duration.ts
@@ -86,12 +86,21 @@ export class Duration {
 
 export function applyDuration(date: Date | number, duration: Duration): Date {
   const r = new Date(date)
-  r.setFullYear(r.getFullYear() + duration.years)
-  r.setMonth(r.getMonth() + duration.months)
-  r.setDate(r.getDate() + duration.weeks * 7 + duration.days)
-  r.setHours(r.getHours() + duration.hours)
-  r.setMinutes(r.getMinutes() + duration.minutes)
-  r.setSeconds(r.getSeconds() + duration.seconds)
+  if (duration.sign < 0) {
+    r.setUTCSeconds(r.getUTCSeconds() + duration.seconds)
+    r.setUTCMinutes(r.getUTCMinutes() + duration.minutes)
+    r.setUTCHours(r.getUTCHours() + duration.hours)
+    r.setUTCDate(r.getUTCDate() + duration.weeks * 7 + duration.days)
+    r.setUTCMonth(r.getUTCMonth() + duration.months)
+    r.setUTCFullYear(r.getUTCFullYear() + duration.years)
+  } else {
+    r.setUTCFullYear(r.getUTCFullYear() + duration.years)
+    r.setUTCMonth(r.getUTCMonth() + duration.months)
+    r.setUTCDate(r.getUTCDate() + duration.weeks * 7 + duration.days)
+    r.setUTCHours(r.getUTCHours() + duration.hours)
+    r.setUTCMinutes(r.getUTCMinutes() + duration.minutes)
+    r.setUTCSeconds(r.getUTCSeconds() + duration.seconds)
+  }
   return r
 }
 
diff --git a/src/relative-time-element.ts b/src/relative-time-element.ts
index 2130065..068183d 100644
--- a/src/relative-time-element.ts
+++ b/src/relative-time-element.ts
@@ -82,11 +82,12 @@ export class RelativeTimeElement extends HTMLElement implements Intl.DateTimeFor
   #updating: false | Promise<void> = false
 
   get #lang() {
-    return (
-      this.closest('[lang]')?.getAttribute('lang') ||
-      this.ownerDocument.documentElement.getAttribute('lang') ||
-      'default'
-    )
+    const lang = this.closest('[lang]')?.getAttribute('lang') || this.ownerDocument.documentElement.getAttribute('lang')
+    try {
+      return new Intl.Locale(lang ?? '').toString()
+    } catch {
+      return 'default'
+    }
   }
 
   #renderRoot: Node = this.shadowRoot ? this.shadowRoot : this.attachShadow ? this.attachShadow({mode: 'open'}) : this
diff --git a/test/duration.ts b/test/duration.ts
index 3b35600..ecb9e68 100644
--- a/test/duration.ts
+++ b/test/duration.ts
@@ -81,16 +81,19 @@ suite('duration', function () {
   })
 
   suite('applyDuration', function () {
-    const referenceDate = '2022-10-21T16:48:44.104Z'
     const tests = new Set([
-      {input: 'P4Y', expected: '2026-10-21T16:48:44.104Z'},
-      {input: '-P4Y', expected: '2018-10-21T16:48:44.104Z'},
-      {input: '-P3MT5M', expected: '2022-07-21T16:43:44.104Z'},
-      {input: 'P1Y2M3DT4H5M6S', expected: '2023-12-24T20:53:50.104Z'},
-      {input: 'P5W', expected: '2022-11-25T16:48:44.104Z'},
-      {input: '-P5W', expected: '2022-09-16T16:48:44.104Z'},
+      {referenceDate: '2022-10-21T16:48:44.104Z', input: 'P5W', expected: '2022-11-25T16:48:44.104Z'},
+      {referenceDate: '2022-11-25T16:48:44.104Z', input: '-P5W', expected: '2022-10-21T16:48:44.104Z'},
+      {referenceDate: '2022-07-21T16:43:44.104Z', input: 'P3MT5M', expected: '2022-10-21T16:48:44.104Z'},
+      {referenceDate: '2022-10-21T16:48:44.104Z', input: '-P3MT5M', expected: '2022-07-21T16:43:44.104Z'},
+      {referenceDate: '2022-10-21T16:48:44.104Z', input: 'P4Y', expected: '2026-10-21T16:48:44.104Z'},
+      {referenceDate: '2026-10-21T16:48:44.104Z', input: '-P4Y', expected: '2022-10-21T16:48:44.104Z'},
+      {referenceDate: '2022-10-21T16:48:44.104Z', input: 'P1Y2M3DT4H5M6S', expected: '2023-12-24T20:53:50.104Z'},
+      {referenceDate: '2023-12-24T20:53:50.104Z', input: '-P1Y2M3DT4H5M6S', expected: '2022-10-21T16:48:44.104Z'},
+      {referenceDate: '2023-08-15T00:00:00.000Z', input: 'P1Y3M25D', expected: '2024-12-10T00:00:00.000Z'},
+      {referenceDate: '2024-12-10T00:00:00.000Z', input: '-P1Y3M25D', expected: '2023-08-15T00:00:00.000Z'},
     ])
-    for (const {input, expected} of tests) {
+    for (const {referenceDate, input, expected} of tests) {
       test(`${referenceDate} -> ${input} -> ${expected}`, () => {
         assert.equal(applyDuration(new Date(referenceDate), Duration.from(input))?.toISOString(), expected)
       })
diff --git a/test/relative-time.js b/test/relative-time.js
index b00e592..c98a1ae 100644
--- a/test/relative-time.js
+++ b/test/relative-time.js
@@ -434,6 +434,17 @@ suite('relative-time', function () {
     })
   }
 
+  test('renders correctly when given an invalid lang', async () => {
+    const now = new Date().toISOString()
+
+    const element = document.createElement('relative-time')
+    element.setAttribute('datetime', now)
+    element.setAttribute('lang', 'does-not-exist')
+
+    await Promise.resolve()
+    assert.equal(element.shadowRoot.textContent, 'now')
+  })
+
   suite('[tense=past]', function () {
     test('always uses relative dates', async () => {
       freezeTime(new Date(2033, 1, 1))
@@ -517,7 +528,7 @@ suite('relative-time', function () {
       time.setAttribute('datetime', datetime)
       time.setAttribute('format', 'micro')
       await Promise.resolve()
-      assert.equal(time.shadowRoot.textContent, '10y')
+      assert.equal(time.shadowRoot.textContent, '11y')
     })
 
     test('micro formats future times', async () => {