diff --git a/backend/__tests__/routes/adminUsers.dateNormalize.test.js b/backend/__tests__/routes/adminUsers.dateNormalize.test.js
new file mode 100644
index 00000000..d30d1320
--- /dev/null
+++ b/backend/__tests__/routes/adminUsers.dateNormalize.test.js
@@ -0,0 +1,123 @@
+/**
+ * Pin the date-field normalisation in adminUsers transformer (#485).
+ *
+ * The Users page crashed on native/SQLite installs because Postgres
+ * returned ISO strings while SQLite returned epoch-millisecond
+ * integers, and the frontend `parseISO()` blew up on numbers with
+ * "e.split is not a function". The transformer now coerces every
+ * shape to an ISO 8601 string before serialising.
+ *
+ * These tests guard the contract so a future refactor can't quietly
+ * regress and re-break the same page on the same DB.
+ */
+
+const adminUsersRoute = require('../../src/routes/adminUsers');
+const { toIso, transformUser, transformInvitation } = adminUsersRoute.__test;
+
+describe('toIso', () => {
+  it('passes null and undefined through unchanged', () => {
+    expect(toIso(null)).toBeNull();
+    expect(toIso(undefined)).toBeUndefined();
+    // Empty string also short-circuits — important so an unset
+    // last_login renders as "Never" instead of 1970-01-01T00:00:00Z.
+    expect(toIso('')).toBe('');
+  });
+
+  it('coerces an integer epoch (SQLite shape) to an ISO 8601 string', () => {
+    // 2026-05-14T10:00:00.000Z, in epoch ms.
+    const epochMs = 1778752800000;
+    expect(toIso(epochMs)).toBe('2026-05-14T10:00:00.000Z');
+  });
+
+  it('coerces a stringified large integer to an ISO 8601 string', () => {
+    // Some SQLite drivers stringify large integers because they
+    // overflow JS safe-integer in the driver's serialiser. Re-coerce
+    // so the frontend doesn't try to parseISO('1778752800000').
+    expect(toIso('1778752800000')).toBe('2026-05-14T10:00:00.000Z');
+  });
+
+  it('coerces a Date instance via toISOString', () => {
+    const d = new Date('2026-01-01T12:34:56.000Z');
+    expect(toIso(d)).toBe('2026-01-01T12:34:56.000Z');
+  });
+
+  it('passes an existing ISO string through unchanged', () => {
+    const iso = '2026-05-14T10:00:00.000Z';
+    expect(toIso(iso)).toBe(iso);
+  });
+
+  it('passes a non-numeric short string (e.g. truncated date) through unchanged', () => {
+    // Defensive: anything that isn't a 10+ digit integer string is
+    // treated as already-stringified — the date library will surface
+    // the failure cleanly if it's malformed, rather than the
+    // transformer silently rewriting it.
+    expect(toIso('2026-05-14')).toBe('2026-05-14');
+  });
+});
+
+describe('transformUser', () => {
+  it('normalises last_login, created_at, updated_at coming from SQLite', () => {
+    const sqliteRow = {
+      id: 1,
+      username: 'admin',
+      email: 'admin@example.com',
+      is_active: 1,
+      last_login: 1778752800000,            // epoch ms
+      last_login_ip: '127.0.0.1',
+      created_at: 1778751144600,            // epoch ms
+      updated_at: 1778751242320,            // epoch ms
+      role_id: 1,
+      role_name: 'super_admin',
+      role_display_name: 'Super Admin',
+      created_by_username: null,
+    };
+
+    const out = transformUser(sqliteRow);
+
+    expect(out.lastLogin).toBe('2026-05-14T10:00:00.000Z');
+    expect(out.createdAt).toMatch(/^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}\.\d{3}Z$/);
+    expect(out.updatedAt).toMatch(/^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}\.\d{3}Z$/);
+    // Other fields untouched.
+    expect(out.username).toBe('admin');
+    expect(out.lastLoginIp).toBe('127.0.0.1');
+  });
+
+  it('leaves Postgres ISO strings intact', () => {
+    const pgRow = {
+      id: 2,
+      username: 'second',
+      email: 'second@example.com',
+      is_active: true,
+      last_login: '2026-05-14T10:00:00.000Z',
+      created_at: '2026-05-13T08:00:00.000Z',
+      updated_at: '2026-05-14T09:00:00.000Z',
+    };
+    const out = transformUser(pgRow);
+    expect(out.lastLogin).toBe('2026-05-14T10:00:00.000Z');
+    expect(out.createdAt).toBe('2026-05-13T08:00:00.000Z');
+    expect(out.updatedAt).toBe('2026-05-14T09:00:00.000Z');
+  });
+
+  it('keeps last_login null when the user has never logged in', () => {
+    const out = transformUser({
+      id: 3, username: 'fresh', email: 'fresh@example.com',
+      is_active: 1, last_login: null,
+    });
+    expect(out.lastLogin).toBeNull();
+  });
+});
+
+describe('transformInvitation', () => {
+  it('normalises expires_at and created_at from SQLite epoch-ms', () => {
+    const out = transformInvitation({
+      id: 9,
+      email: 'invitee@example.com',
+      expires_at: 1779357600000,
+      created_at: 1778752800000,
+      role_name: 'admin',
+      invited_by: 'admin',
+    });
+    expect(out.expiresAt).toMatch(/^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}\.\d{3}Z$/);
+    expect(out.createdAt).toBe('2026-05-14T10:00:00.000Z');
+  });
+});
diff --git a/backend/src/routes/adminUsers.js b/backend/src/routes/adminUsers.js
index a236b175..1852f903 100644
--- a/backend/src/routes/adminUsers.js
+++ b/backend/src/routes/adminUsers.js
@@ -11,6 +11,37 @@ const { handleAsync, validateRequest, successResponse } = require('../utils/rout
 const userManagementService = require('../services/userManagementService');
 const router = express.Router();
 
+/**
+ * Coerce any of the shapes a TIMESTAMP column produces across our
+ * supported drivers into a single ISO 8601 string the frontend (and
+ * any external API consumer) can safely pass to date-fns / new Date.
+ *
+ * Postgres → Date object (becomes ISO via JSON.stringify anyway, but
+ *   pinning the format defends against driver-side surprises).
+ * SQLite → integer milliseconds since epoch (the surface that crashed
+ *   the admin Users page in #485 — `parseISO(123456789)` blows up
+ *   with "e.split is not a function"). Native installs default to
+ *   SQLite, so this path matters every release.
+ * Already a string → assume it's a parseable ISO/RFC3339 (Postgres
+ *   driver may stringify under JSON serialization mid-pipeline).
+ *
+ * Returns null/undefined unchanged so an unset last_login surfaces as
+ * "Never" in the UI rather than 1970-01-01T00:00:00Z.
+ */
+function toIso(value) {
+  if (value === null || value === undefined || value === '') return value;
+  if (value instanceof Date) return value.toISOString();
+  if (typeof value === 'number') return new Date(value).toISOString();
+  if (typeof value === 'string') {
+    // Numeric-as-string ("1778752458666") happens when the SQLite
+    // driver stringifies large integers — re-coerce so the frontend
+    // doesn't try to parseISO('1778752458666').
+    if (/^\d{10,}$/.test(value)) return new Date(Number(value)).toISOString();
+    return value;
+  }
+  return value;
+}
+
 /**
  * Transform user object from snake_case (DB) to camelCase (API)
  */
@@ -20,10 +51,10 @@ function transformUser(user) {
     username: user.username,
     email: user.email,
     isActive: user.is_active,
-    lastLogin: user.last_login,
+    lastLogin: toIso(user.last_login),
     lastLoginIp: user.last_login_ip,
-    createdAt: user.created_at,
-    updatedAt: user.updated_at,
+    createdAt: toIso(user.created_at),
+    updatedAt: toIso(user.updated_at),
     roleId: user.role_id,
     roleName: user.role_name,
     roleDisplayName: user.role_display_name,
@@ -52,8 +83,8 @@ function transformInvitation(invitation) {
   return {
     id: invitation.id,
     email: invitation.email,
-    expiresAt: invitation.expires_at,
-    createdAt: invitation.created_at,
+    expiresAt: toIso(invitation.expires_at),
+    createdAt: toIso(invitation.created_at),
     roleName: invitation.role_name,
     invitedBy: invitation.invited_by
   };
@@ -212,4 +243,7 @@ router.post('/:id/reset-password', [
   successResponse(res, { message: 'Password reset email sent', ...result });
 }));
 
+// Test surface: expose the date normaliser so the unit test can pin
+// the contract without spinning up the full router.
 module.exports = router;
+module.exports.__test = { toIso, transformUser, transformInvitation };
diff --git a/frontend/src/services/userManagement.service.ts b/frontend/src/services/userManagement.service.ts
index 3f62afd9..76087d7e 100644
--- a/frontend/src/services/userManagement.service.ts
+++ b/frontend/src/services/userManagement.service.ts
@@ -1,6 +1,33 @@
 import { api } from '../config/api';
 import type { AdminUser, AdminRole, AdminInvitation } from '../types';
 
+/**
+ * Defence-in-depth normalisation for date fields (#485). The backend
+ * now always returns these as ISO strings via toIso() in adminUsers.js,
+ * but this wrapper:
+ *
+ *   - Lets the page survive a stale backend during a mid-deploy window
+ *     (older backend still returns SQLite epoch-ms numbers).
+ *   - Lets the page survive an external API consumer's response cache
+ *     that captured the pre-fix shape.
+ *
+ * The original crash was `parseISO(123456789)` in
+ * AdminUsersPage → `e.split is not a function`. Coercing here means
+ * the next consumer of the AdminUser type can rely on the field
+ * being a string regardless of how it landed.
+ */
+function normalizeDateValue(value: unknown): string | null | undefined {
+  if (value === null || value === undefined || value === '') return value as null | undefined;
+  if (value instanceof Date) return value.toISOString();
+  if (typeof value === 'number') return new Date(value).toISOString();
+  if (typeof value === 'string') {
+    // SQLite via some drivers stringifies large integers — coerce back.
+    if (/^\d{10,}$/.test(value)) return new Date(Number(value)).toISOString();
+    return value;
+  }
+  return undefined;
+}
+
 // Transform snake_case API response to camelCase for frontend
 // eslint-disable-next-line @typescript-eslint/no-explicit-any
 function transformUser(user: any): AdminUser {
@@ -9,10 +36,10 @@ function transformUser(user: any): AdminUser {
     username: user.username,
     email: user.email,
     isActive: user.isActive ?? user.is_active,
-    lastLogin: user.lastLogin ?? user.last_login,
+    lastLogin: normalizeDateValue(user.lastLogin ?? user.last_login),
     lastLoginIp: user.lastLoginIp ?? user.last_login_ip,
-    createdAt: user.createdAt ?? user.created_at,
-    updatedAt: user.updatedAt ?? user.updated_at,
+    createdAt: normalizeDateValue(user.createdAt ?? user.created_at),
+    updatedAt: normalizeDateValue(user.updatedAt ?? user.updated_at),
     roleId: user.roleId ?? user.role_id,
     roleName: user.roleName ?? user.role_name,
     roleDisplayName: user.roleDisplayName ?? user.role_display_name,
@@ -20,6 +47,18 @@ function transformUser(user: any): AdminUser {
   };
 }
 
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+function transformInvitation(invitation: any): AdminInvitation {
+  return {
+    id: invitation.id,
+    email: invitation.email,
+    roleName: invitation.roleName ?? invitation.role_name,
+    invitedBy: invitation.invitedBy ?? invitation.invited_by,
+    expiresAt: normalizeDateValue(invitation.expiresAt ?? invitation.expires_at) as string,
+    createdAt: normalizeDateValue(invitation.createdAt ?? invitation.created_at) as string,
+  };
+}
+
 // eslint-disable-next-line @typescript-eslint/no-explicit-any
 function transformRole(role: any): AdminRole {
   return {
@@ -127,7 +166,7 @@ export const userManagementService = {
    */
   async getInvitations(): Promise<AdminInvitation[]> {
     const response = await api.get<GetInvitationsResponse>('/admin/users/invitations');
-    return response.data.invitations;
+    return response.data.invitations.map(transformInvitation);
   },
 
   /**
@@ -135,7 +174,7 @@ export const userManagementService = {
    */
   async createInvitation(data: CreateInvitationData): Promise<AdminInvitation> {
     const response = await api.post<CreateInvitationResponse>('/admin/users/invite', data);
-    return response.data.invitation;
+    return transformInvitation(response.data.invitation);
   },
 
   /**