diff --git a/app/Http/Controllers/Auth/RegisteredUserController.php b/app/Http/Controllers/Auth/RegisteredUserController.php
index 746b0704..85284ffb 100644
--- a/app/Http/Controllers/Auth/RegisteredUserController.php
+++ b/app/Http/Controllers/Auth/RegisteredUserController.php
@@ -6,6 +6,7 @@
 
 use App\Enums\User\Setup;
 use App\Http\Controllers\Controller;
+use App\Rules\Timezone;
 use App\Models\Language;
 use App\Models\User;
 use App\Models\Workspace;
@@ -40,7 +41,7 @@ public function store(Request $request): RedirectResponse
             'name' => ['required', 'string', 'max:255'],
             'email' => ['required', 'string', 'lowercase', 'email', 'max:255', 'unique:'.User::class],
             'password' => ['required', Rules\Password::defaults()],
-            'timezone' => ['nullable', 'string', 'timezone'],
+            'timezone' => ['nullable', 'string', new Timezone],
         ]);
 
         // Check if registering via invite link (redirect contains /invites/)
diff --git a/app/Http/Requests/UpdateWorkspaceRequest.php b/app/Http/Requests/UpdateWorkspaceRequest.php
index fd7b2ff8..a96bf0fb 100644
--- a/app/Http/Requests/UpdateWorkspaceRequest.php
+++ b/app/Http/Requests/UpdateWorkspaceRequest.php
@@ -2,6 +2,7 @@
 
 namespace App\Http\Requests;
 
+use App\Rules\Timezone;
 use Illuminate\Foundation\Http\FormRequest;
 
 class UpdateWorkspaceRequest extends FormRequest
@@ -15,7 +16,7 @@ public function rules(): array
     {
         return [
             'name' => ['required', 'string', 'max:255'],
-            'timezone' => ['required', 'string', 'timezone:all'],
+            'timezone' => ['required', 'string', new Timezone],
         ];
     }
 
diff --git a/app/Rules/Timezone.php b/app/Rules/Timezone.php
new file mode 100644
index 00000000..90301db0
--- /dev/null
+++ b/app/Rules/Timezone.php
@@ -0,0 +1,23 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Rules;
+
+use Closure;
+use Illuminate\Contracts\Validation\ValidationRule;
+
+class Timezone implements ValidationRule
+{
+    /**
+     * @param  \Closure(string, ?string=): \Illuminate\Translation\PotentiallyTranslatedString  $fail
+     */
+    public function validate(string $attribute, mixed $value, Closure $fail): void
+    {
+        try {
+            new \DateTimeZone($value);
+        } catch (\Exception) {
+            $fail('The :attribute must be a valid timezone.');
+        }
+    }
+}
diff --git a/tests/Feature/Auth/RegistrationTest.php b/tests/Feature/Auth/RegistrationTest.php
index 449799da..3a5924e1 100644
--- a/tests/Feature/Auth/RegistrationTest.php
+++ b/tests/Feature/Auth/RegistrationTest.php
@@ -61,6 +61,46 @@
     expect($user->email_verified_at)->toBeNull();
 });
 
+test('new users can register with deprecated timezone Asia/Calcutta', function () {
+    $this->post(route('register.store'), [
+        'name' => 'Test User',
+        'email' => 'test@example.com',
+        'password' => 'Password123!',
+        'timezone' => 'Asia/Calcutta',
+    ]);
+
+    $user = User::where('email', 'test@example.com')->first();
+
+    expect($user)->not->toBeNull();
+    expect($user->workspaces->first()->timezone)->toBe('Asia/Calcutta');
+});
+
+test('new users can register with deprecated timezone US/Eastern', function () {
+    $this->post(route('register.store'), [
+        'name' => 'Test User',
+        'email' => 'test@example.com',
+        'password' => 'Password123!',
+        'timezone' => 'US/Eastern',
+    ]);
+
+    $user = User::where('email', 'test@example.com')->first();
+
+    expect($user)->not->toBeNull();
+    expect($user->workspaces->first()->timezone)->toBe('US/Eastern');
+});
+
+test('new users cannot register with invalid timezone', function () {
+    $response = $this->post(route('register.store'), [
+        'name' => 'Test User',
+        'email' => 'test@example.com',
+        'password' => 'Password123!',
+        'timezone' => 'Invalid/Timezone',
+    ]);
+
+    $response->assertSessionHasErrors('timezone');
+    expect(User::where('email', 'test@example.com')->exists())->toBeFalse();
+});
+
 test('new users registering via invite have verified email automatically', function () {
     $this->post(route('register.store'), [
         'name' => 'Test User',
diff --git a/tests/Unit/Rules/TimezoneRuleTest.php b/tests/Unit/Rules/TimezoneRuleTest.php
new file mode 100644
index 00000000..2440bb39
--- /dev/null
+++ b/tests/Unit/Rules/TimezoneRuleTest.php
@@ -0,0 +1,46 @@
+<?php
+
+use App\Rules\Timezone;
+use Illuminate\Support\Facades\Validator;
+
+test('accepts valid timezone', function () {
+    $validator = Validator::make(['tz' => 'America/Sao_Paulo'], ['tz' => new Timezone]);
+
+    expect($validator->passes())->toBeTrue();
+});
+
+test('accepts UTC', function () {
+    $validator = Validator::make(['tz' => 'UTC'], ['tz' => new Timezone]);
+
+    expect($validator->passes())->toBeTrue();
+});
+
+test('accepts deprecated timezone Asia/Calcutta', function () {
+    $validator = Validator::make(['tz' => 'Asia/Calcutta'], ['tz' => new Timezone]);
+
+    expect($validator->passes())->toBeTrue();
+});
+
+test('accepts deprecated timezone US/Eastern', function () {
+    $validator = Validator::make(['tz' => 'US/Eastern'], ['tz' => new Timezone]);
+
+    expect($validator->passes())->toBeTrue();
+});
+
+test('rejects invalid timezone', function () {
+    $validator = Validator::make(['tz' => 'Invalid/Timezone'], ['tz' => new Timezone]);
+
+    expect($validator->passes())->toBeFalse();
+});
+
+test('rejects random string', function () {
+    $validator = Validator::make(['tz' => 'blabla'], ['tz' => new Timezone]);
+
+    expect($validator->passes())->toBeFalse();
+});
+
+test('accepts Europe/London', function () {
+    $validator = Validator::make(['tz' => 'Europe/London'], ['tz' => new Timezone]);
+
+    expect($validator->passes())->toBeTrue();
+});