diff --git a/src/app/u/[handle]/page.tsx b/src/app/u/[handle]/page.tsx
index 2700b45..ec0bd89 100644
--- a/src/app/u/[handle]/page.tsx
+++ b/src/app/u/[handle]/page.tsx
@@ -15,6 +15,7 @@ import { ProfileHeader } from '@/components/profile/profile-header'
 import { ProfileSkeleton } from '@/components/profile/profile-skeleton'
 import { getUserProfile, getPublicSettings } from '@/lib/api/client'
 import { useAuth } from '@/hooks/use-auth'
+import { formatDateLong } from '@/lib/format'
 import type { UserProfile } from '@/lib/api/types'
 
 interface UserProfilePageProps {
@@ -119,11 +120,7 @@ export default function UserProfilePage({ params }: UserProfilePageProps) {
   const reputationScore = computeReputation(profile.activity)
   const postCount = profile.activity.topicCount + profile.activity.replyCount
 
-  const joinDate = new Date(profile.firstSeenAt).toLocaleDateString('en-US', {
-    year: 'numeric',
-    month: 'long',
-    day: 'numeric',
-  })
+  const joinDate = formatDateLong(profile.firstSeenAt)
 
   return (
     <ForumLayout>
diff --git a/src/components/admin/users/user-card.tsx b/src/components/admin/users/user-card.tsx
index eeaa30f..c2de8bb 100644
--- a/src/components/admin/users/user-card.tsx
+++ b/src/components/admin/users/user-card.tsx
@@ -7,6 +7,7 @@
 
 import { Prohibit, WarningCircle } from '@phosphor-icons/react'
 import { cn } from '@/lib/utils'
+import { formatDateShort } from '@/lib/format'
 import type { AdminUser } from '@/lib/api/types'
 
 const ROLE_COLORS: Record<string, string> = {
@@ -15,14 +16,6 @@ const ROLE_COLORS: Record<string, string> = {
   member: 'bg-muted text-muted-foreground',
 }
 
-function formatDate(dateStr: string) {
-  return new Date(dateStr).toLocaleDateString('en-US', {
-    month: 'short',
-    day: 'numeric',
-    year: 'numeric',
-  })
-}
-
 interface UserCardProps {
   user: AdminUser
   onBan: (did: string) => void
@@ -61,7 +54,7 @@ export function UserCard({ user, onBan, onUnban }: UserCardProps) {
             <span>{user.topicCount} topics</span>
             <span>{user.replyCount} replies</span>
             <span>{user.reportCount} reports</span>
-            <span>Joined {formatDate(user.firstSeenAt)}</span>
+            <span>Joined {formatDateShort(user.firstSeenAt)}</span>
           </div>
           {user.bannedFromOtherCommunities > 0 && (
             <p className="mt-1 inline-flex items-center gap-1 text-xs font-medium text-destructive">
diff --git a/src/components/profile/profile-header.test.tsx b/src/components/profile/profile-header.test.tsx
index 85624d4..c77427f 100644
--- a/src/components/profile/profile-header.test.tsx
+++ b/src/components/profile/profile-header.test.tsx
@@ -290,18 +290,22 @@ describe('ProfileHeader', () => {
   })
 
   it('renders AT Protocol account creation date when accountCreatedAt is set', () => {
-    render(
+    const { container } = render(
       <ProfileHeader
         profile={createProfile({ accountCreatedAt: '2024-06-15T10:30:00.000Z' })}
         {...defaultProps}
       />
     )
-    expect(screen.getByText(/since June 15, 2024/i)).toBeInTheDocument()
+    const timeEl = container.querySelector('time[datetime="2024-06-15T10:30:00.000Z"]')
+    expect(timeEl).toBeInTheDocument()
+    expect(timeEl).toHaveTextContent('June 15, 2024')
   })
 
   it('does not render AT Protocol account creation date when accountCreatedAt is null', () => {
-    render(<ProfileHeader profile={createProfile({ accountCreatedAt: null })} {...defaultProps} />)
-    expect(screen.queryByText(/since/i)).not.toBeInTheDocument()
+    const { container } = render(
+      <ProfileHeader profile={createProfile({ accountCreatedAt: null })} {...defaultProps} />
+    )
+    expect(container.querySelector('time')).not.toBeInTheDocument()
   })
 
   it('renders labels section when labels are present', () => {
diff --git a/src/components/profile/profile-stats.tsx b/src/components/profile/profile-stats.tsx
index 399e2f5..1acab09 100644
--- a/src/components/profile/profile-stats.tsx
+++ b/src/components/profile/profile-stats.tsx
@@ -6,6 +6,7 @@
 
 import { Users, ArrowSquareOut, CalendarBlank } from '@phosphor-icons/react'
 import { formatCount } from '@/lib/format-count'
+import { formatDateLong } from '@/lib/format'
 import type { UserProfile } from '@/lib/api/types'
 
 interface ProfileStatsProps {
@@ -15,11 +16,7 @@ interface ProfileStatsProps {
 
 export function ProfileStats({ profile, handle }: ProfileStatsProps) {
   const accountCreatedDate = profile.accountCreatedAt
-    ? new Date(profile.accountCreatedAt).toLocaleDateString('en-US', {
-        year: 'numeric',
-        month: 'long',
-        day: 'numeric',
-      })
+    ? formatDateLong(profile.accountCreatedAt)
     : null
 
   return (
@@ -42,7 +39,7 @@ export function ProfileStats({ profile, handle }: ProfileStatsProps) {
       {accountCreatedDate && (
         <span className="flex items-center gap-1">
           <CalendarBlank size={16} aria-hidden="true" />
-          Since {accountCreatedDate}
+          Since <time dateTime={profile.accountCreatedAt!}>{accountCreatedDate}</time>
         </span>
       )}
 
diff --git a/src/lib/format.ts b/src/lib/format.ts
index d090907..ead3176 100644
--- a/src/lib/format.ts
+++ b/src/lib/format.ts
@@ -52,6 +52,17 @@ export function formatDateShort(dateStr: string): string {
   })
 }
 
+/**
+ * Formats an ISO date string as a long date (e.g., "January 15, 2026").
+ */
+export function formatDateLong(dateStr: string): string {
+  return new Date(dateStr).toLocaleDateString('en-US', {
+    year: 'numeric',
+    month: 'long',
+    day: 'numeric',
+  })
+}
+
 /**
  * Formats a number with locale-aware separators (e.g., 1,234).
  */
diff --git a/src/mocks/data.ts b/src/mocks/data.ts
index a85c141..9ef5101 100644
--- a/src/mocks/data.ts
+++ b/src/mocks/data.ts
@@ -1036,6 +1036,7 @@ export const mockUserProfiles: Record<string, UserProfile> = {
     role: 'admin',
     firstSeenAt: TWO_DAYS_AGO,
     lastActiveAt: NOW,
+    accountCreatedAt: null,
     followersCount: 150,
     followsCount: 75,
     atprotoPostsCount: 230,
@@ -1065,6 +1066,7 @@ export const mockUserProfiles: Record<string, UserProfile> = {
     role: 'moderator',
     firstSeenAt: TWO_DAYS_AGO,
     lastActiveAt: YESTERDAY,
+    accountCreatedAt: null,
     followersCount: 30,
     followsCount: 20,
     atprotoPostsCount: 45,