diff --git a/app/Exceptions/Api/OverlappingTimeEntryApiException.php b/app/Exceptions/Api/OverlappingTimeEntryApiException.php
new file mode 100644
index 00000000..7c488955
--- /dev/null
+++ b/app/Exceptions/Api/OverlappingTimeEntryApiException.php
@@ -0,0 +1,10 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Exceptions\Api;
+
+class OverlappingTimeEntryApiException extends ApiException
+{
+    public const string KEY = 'overlapping_time_entry';
+}
diff --git a/app/Http/Controllers/Api/V1/OrganizationController.php b/app/Http/Controllers/Api/V1/OrganizationController.php
index e3a0b1eb..a046535b 100644
--- a/app/Http/Controllers/Api/V1/OrganizationController.php
+++ b/app/Http/Controllers/Api/V1/OrganizationController.php
@@ -61,6 +61,9 @@ class OrganizationController extends Controller
         if ($request->getTimeFormat() !== null) {
             $organization->time_format = $request->getTimeFormat();
         }
+        if ($request->getPreventOverlappingTimeEntries() !== null) {
+            $organization->prevent_overlapping_time_entries = $request->getPreventOverlappingTimeEntries();
+        }
         $hasBillableRate = $request->has('billable_rate');
         if ($hasBillableRate) {
             $oldBillableRate = $organization->billable_rate;
diff --git a/app/Http/Controllers/Api/V1/TimeEntryController.php b/app/Http/Controllers/Api/V1/TimeEntryController.php
index a36c9f71..ed1ca073 100644
--- a/app/Http/Controllers/Api/V1/TimeEntryController.php
+++ b/app/Http/Controllers/Api/V1/TimeEntryController.php
@@ -7,6 +7,7 @@ namespace App\Http\Controllers\Api\V1;
 use App\Enums\ExportFormat;
 use App\Enums\Role;
 use App\Exceptions\Api\FeatureIsNotAvailableInFreePlanApiException;
+use App\Exceptions\Api\OverlappingTimeEntryApiException;
 use App\Exceptions\Api\PdfRendererIsNotConfiguredException;
 use App\Exceptions\Api\TimeEntryCanNotBeRestartedApiException;
 use App\Exceptions\Api\TimeEntryStillRunningApiException;
@@ -45,6 +46,7 @@ use Illuminate\Database\Eloquent\Builder;
 use Illuminate\Http\File;
 use Illuminate\Http\JsonResponse;
 use Illuminate\Http\Resources\Json\JsonResource;
+use Illuminate\Support\Carbon;
 use Illuminate\Support\Collection;
 use Illuminate\Support\Facades\Auth;
 use Illuminate\Support\Facades\Blade;
@@ -56,6 +58,43 @@ use Spatie\TemporaryDirectory\TemporaryDirectory;
 
 class TimeEntryController extends Controller
 {
+    private function assertNoOverlap(Organization $organization, Member $member, \Illuminate\Support\Carbon $start, ?\Illuminate\Support\Carbon $end, ?TimeEntry $exclude = null): void
+    {
+        if (! $organization->prevent_overlapping_time_entries) {
+            return;
+        }
+
+        $query = TimeEntry::query()
+            ->where('organization_id', $organization->getKey())
+            ->where('user_id', $member->user_id)
+            ->when($exclude !== null, function (Builder $q) use ($exclude): void {
+                $q->where('id', '!=', $exclude->getKey());
+            })
+            ->where(function (Builder $q) use ($start, $end): void {
+                $q->where(function (Builder $q2) use ($start): void {
+                    $q2->where('end', '>', $start)
+                        ->where('start', '<', $start);
+                });
+
+                if ($end !== null) {
+                    $q->orWhere(function (Builder $q4) use ($end): void {
+                        $q4->where('start', '<', $end)
+                            ->where('end', '>', $end);
+                    });
+                    // Check if the new entry completely surrounds an existing entry
+                    $q->orWhere(function (Builder $q6) use ($start, $end): void {
+                        $q6->where('start', '>=', $start)
+                            ->where('end', '<=', $end);
+                    });
+                }
+
+            });
+
+        if ($query->exists()) {
+            throw new OverlappingTimeEntryApiException;
+        }
+    }
+
     protected function checkPermission(Organization $organization, string $permission, ?TimeEntry $timeEntry = null): void
     {
         parent::checkPermission($organization, $permission);
@@ -549,17 +588,15 @@ class TimeEntryController extends Controller
             throw new TimeEntryStillRunningApiException;
         }
 
+        // Overlap check for create
+        $start = Carbon::parse($request->input('start'));
+        $end = $request->input('end') !== null ? Carbon::parse($request->input('end')) : null;
+        $this->assertNoOverlap($organization, $member, $start, $end);
+
         $project = $request->input('project_id') !== null ? Project::findOrFail((string) $request->input('project_id')) : null;
         $client = $project?->client;
         $task = $request->input('task_id') !== null ? $project->tasks()->findOrFail((string) $request->input('task_id')) : null;
 
-        if ($project !== null) {
-            RecalculateSpentTimeForProject::dispatch($project);
-        }
-        if ($task !== null) {
-            RecalculateSpentTimeForTask::dispatch($task);
-        }
-
         $timeEntry = new TimeEntry;
         $timeEntry->fill($request->validated());
         $timeEntry->client()->associate($client);
@@ -569,6 +606,13 @@ class TimeEntryController extends Controller
         $timeEntry->setComputedAttributeValue('billable_rate');
         $timeEntry->save();
 
+        if ($project !== null) {
+            RecalculateSpentTimeForProject::dispatch($project);
+        }
+        if ($task !== null) {
+            RecalculateSpentTimeForTask::dispatch($task);
+        }
+
         return new TimeEntryResource($timeEntry);
     }
 
@@ -593,6 +637,13 @@ class TimeEntryController extends Controller
             throw new TimeEntryCanNotBeRestartedApiException;
         }
 
+        // Overlap check for update (exclude current)
+        /** @var Member $effectiveMember */
+        $effectiveMember = $request->has('member_id') ? Member::query()->findOrFail($request->input('member_id')) : $timeEntry->member;
+        $effectiveStart = $request->has('start') ? Carbon::parse($request->input('start')) : $timeEntry->start;
+        $effectiveEnd = $request->has('end') ? ($request->input('end') !== null ? Carbon::parse($request->input('end')) : null) : $timeEntry->end;
+        $this->assertNoOverlap($organization, $effectiveMember, $effectiveStart, $effectiveEnd, $timeEntry);
+
         $oldProject = $timeEntry->project;
         $oldTask = $timeEntry->task;
 
diff --git a/app/Http/Requests/V1/Organization/OrganizationUpdateRequest.php b/app/Http/Requests/V1/Organization/OrganizationUpdateRequest.php
index 074c4b80..042fcfc0 100644
--- a/app/Http/Requests/V1/Organization/OrganizationUpdateRequest.php
+++ b/app/Http/Requests/V1/Organization/OrganizationUpdateRequest.php
@@ -39,6 +39,9 @@ class OrganizationUpdateRequest extends BaseFormRequest
             'employees_can_see_billable_rates' => [
                 'boolean',
             ],
+            'prevent_overlapping_time_entries' => [
+                'boolean',
+            ],
             'number_format' => [
                 Rule::enum(NumberFormat::class),
             ],
@@ -98,4 +101,9 @@ class OrganizationUpdateRequest extends BaseFormRequest
     {
         return $this->has('employees_can_see_billable_rates') ? $this->boolean('employees_can_see_billable_rates') : null;
     }
+
+    public function getPreventOverlappingTimeEntries(): ?bool
+    {
+        return $this->has('prevent_overlapping_time_entries') ? $this->boolean('prevent_overlapping_time_entries') : null;
+    }
 }
diff --git a/app/Http/Resources/V1/Organization/OrganizationResource.php b/app/Http/Resources/V1/Organization/OrganizationResource.php
index f6ceee74..e7baa9e9 100644
--- a/app/Http/Resources/V1/Organization/OrganizationResource.php
+++ b/app/Http/Resources/V1/Organization/OrganizationResource.php
@@ -53,6 +53,8 @@ class OrganizationResource extends BaseResource
             'billable_rate' => $this->showBillableRate ? $this->resource->billable_rate : null,
             /** @var bool $employees_can_see_billable_rates Can members of the organization with role "employee" see the billable rates */
             'employees_can_see_billable_rates' => $this->resource->employees_can_see_billable_rates,
+            /** @var bool $prevent_overlapping_time_entries Prevent creating overlapping time entries (only new entries) */
+            'prevent_overlapping_time_entries' => $this->resource->prevent_overlapping_time_entries,
             /** @var string $currency Currency code (ISO 4217) */
             'currency' => $this->resource->currency,
             /** @var string $currency_symbol Currency symbol */
diff --git a/app/Models/Organization.php b/app/Models/Organization.php
index a8b1f93e..35710e10 100644
--- a/app/Models/Organization.php
+++ b/app/Models/Organization.php
@@ -70,6 +70,7 @@ class Organization extends JetstreamTeam implements AuditableContract
         'personal_team' => 'boolean',
         'currency' => 'string',
         'employees_can_see_billable_rates' => 'boolean',
+        'prevent_overlapping_time_entries' => 'boolean',
         'number_format' => NumberFormat::class,
         'currency_format' => CurrencyFormat::class,
         'date_format' => DateFormat::class,
diff --git a/database/migrations/2025_10_02_000001_add_prevent_overlapping_time_entries_to_organizations_table.php b/database/migrations/2025_10_02_000001_add_prevent_overlapping_time_entries_to_organizations_table.php
new file mode 100644
index 00000000..9157400d
--- /dev/null
+++ b/database/migrations/2025_10_02_000001_add_prevent_overlapping_time_entries_to_organizations_table.php
@@ -0,0 +1,30 @@
+<?php
+
+declare(strict_types=1);
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        Schema::table('organizations', function (Blueprint $table): void {
+            $table->boolean('prevent_overlapping_time_entries')->default(false)->after('employees_can_see_billable_rates');
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::table('organizations', function (Blueprint $table): void {
+            $table->dropColumn('prevent_overlapping_time_entries');
+        });
+    }
+};
diff --git a/lang/en/exceptions.php b/lang/en/exceptions.php
index b5af171d..fda420e6 100644
--- a/lang/en/exceptions.php
+++ b/lang/en/exceptions.php
@@ -14,6 +14,7 @@ use App\Exceptions\Api\OnlyOwnerCanChangeOwnership;
 use App\Exceptions\Api\OnlyPlaceholdersCanBeMergedIntoAnotherMember;
 use App\Exceptions\Api\OrganizationHasNoSubscriptionButMultipleMembersException;
 use App\Exceptions\Api\OrganizationNeedsAtLeastOneOwner;
+use App\Exceptions\Api\OverlappingTimeEntryApiException;
 use App\Exceptions\Api\PdfRendererIsNotConfiguredException;
 use App\Exceptions\Api\PersonalAccessClientIsNotConfiguredException;
 use App\Exceptions\Api\ThisPlaceholderCanNotBeInvitedUseTheMergeToolInsteadException;
@@ -47,6 +48,7 @@ return [
         OnlyPlaceholdersCanBeMergedIntoAnotherMember::KEY => 'Only placeholders can be merged into another member',
         ThisPlaceholderCanNotBeInvitedUseTheMergeToolInsteadException::KEY => 'This placeholder can not be invited use the merge tool instead',
         InvitationForTheEmailAlreadyExistsApiException::KEY => 'The email has already been invited to the organization. Please wait for the user to accept the invitation or resend the invitation email.',
+        OverlappingTimeEntryApiException::KEY => 'Overlapping time entries are not allowed.',
     ],
     'unknown_error_in_admin_panel' => 'An unknown error occurred. Please check the logs.',
 ];
diff --git a/lang/en/validation.php b/lang/en/validation.php
index 61df3046..17baace2 100644
--- a/lang/en/validation.php
+++ b/lang/en/validation.php
@@ -203,6 +203,7 @@ return [
     'organization' => 'The :attribute does not exist.',
     'task_belongs_to_project' => 'The :attribute is not part of the given project.',
     'project_name_already_exists' => 'A project with the same name and client already exists in the organization.',
+    'overlapping_time_entry' => 'Overlapping time entries are not allowed.',
     'tag_name_already_exists' => 'A tag with the same name already exists in the organization.',
     'client_name_already_exists' => 'A client with the same name already exists in the organization.',
     'task_name_already_exists' => 'A task with the same name already exists in the project.',
diff --git a/resources/js/Pages/Teams/Partials/OrganizationFormatSettings.vue b/resources/js/Pages/Teams/Partials/OrganizationFormatSettings.vue
index 45dba3e0..24f1c71e 100644
--- a/resources/js/Pages/Teams/Partials/OrganizationFormatSettings.vue
+++ b/resources/js/Pages/Teams/Partials/OrganizationFormatSettings.vue
@@ -27,7 +27,7 @@ interface FormValues {
 }
 
 const store = useOrganizationStore();
-const { fetchOrganization, updateOrganization } = store;
+const { updateOrganization } = store;
 const { organization } = storeToRefs(store);
 const queryClient = useQueryClient();
 
@@ -47,7 +47,6 @@ const mutation = useMutation({
 });
 
 onMounted(async () => {
-    await fetchOrganization();
     if (organization.value) {
         form.value = {
             number_format: organization.value.number_format as NumberFormat,
diff --git a/resources/js/Pages/Teams/Partials/OrganizationTimeEntrySettings.vue b/resources/js/Pages/Teams/Partials/OrganizationTimeEntrySettings.vue
new file mode 100644
index 00000000..ff7264e7
--- /dev/null
+++ b/resources/js/Pages/Teams/Partials/OrganizationTimeEntrySettings.vue
@@ -0,0 +1,68 @@
+<script setup lang="ts">
+import FormSection from '@/Components/FormSection.vue';
+import PrimaryButton from '@/packages/ui/src/Buttons/PrimaryButton.vue';
+import { onMounted, ref } from 'vue';
+import InputLabel from '@/packages/ui/src/Input/InputLabel.vue';
+import { Checkbox } from '@/packages/ui/src';
+import type { UpdateOrganizationBody } from '@/packages/api/src';
+import { useOrganizationStore } from '@/utils/useOrganization';
+import { storeToRefs } from 'pinia';
+import { useMutation, useQueryClient } from '@tanstack/vue-query';
+
+const store = useOrganizationStore();
+const { updateOrganization } = store;
+const { organization } = storeToRefs(store);
+const queryClient = useQueryClient();
+
+const form = ref<{ prevent_overlapping_time_entries: boolean }>({
+    prevent_overlapping_time_entries: false,
+});
+
+onMounted(async () => {
+    form.value.prevent_overlapping_time_entries =
+        organization.value?.prevent_overlapping_time_entries ?? false;
+});
+
+const mutation = useMutation({
+    mutationFn: (values: Partial<UpdateOrganizationBody>) => updateOrganization(values),
+    onSuccess: () => {
+        queryClient.invalidateQueries({ queryKey: ['organization'] });
+    },
+});
+
+async function submit() {
+    await mutation.mutateAsync({
+        prevent_overlapping_time_entries: form.value.prevent_overlapping_time_entries,
+    });
+}
+</script>
+
+<template>
+    <FormSection>
+        <template #title>Time Entry Settings</template>
+        <template #description>
+            Disallow overlapping time entries for members of this organization. When enabled, users
+            cannot create new time entries that overlap with their existing ones. This only affects
+            newly created entries.
+        </template>
+
+        <template #form>
+            <div class="col-span-6">
+                <div class="col-span-6 sm:col-span-4">
+                    <div class="flex items-center space-x-2">
+                        <Checkbox
+                            id="preventOverlappingTimeEntries"
+                            v-model:checked="form.prevent_overlapping_time_entries" />
+                        <InputLabel
+                            for="preventOverlappingTimeEntries"
+                            value="Prevent overlapping time entries (new entries only)" />
+                    </div>
+                </div>
+            </div>
+        </template>
+
+        <template #actions>
+            <PrimaryButton :disabled="mutation.isPending.value" @click="submit">Save</PrimaryButton>
+        </template>
+    </FormSection>
+</template>
diff --git a/resources/js/Pages/Teams/Show.vue b/resources/js/Pages/Teams/Show.vue
index d98f8738..2bb47bac 100644
--- a/resources/js/Pages/Teams/Show.vue
+++ b/resources/js/Pages/Teams/Show.vue
@@ -8,12 +8,25 @@ import type { Permissions, Role } from '@/types/jetstream';
 import { canUpdateOrganization } from '@/utils/permissions';
 import OrganizationBillableRate from '@/Pages/Teams/Partials/OrganizationBillableRate.vue';
 import OrganizationFormatSettings from '@/Pages/Teams/Partials/OrganizationFormatSettings.vue';
+import OrganizationTimeEntrySettings from '@/Pages/Teams/Partials/OrganizationTimeEntrySettings.vue';
+import { onMounted, ref } from 'vue';
+import { useOrganizationStore } from '@/utils/useOrganization';
+import { storeToRefs } from 'pinia';
 
 defineProps<{
     team: Organization;
     availableRoles: Role[];
     permissions: Permissions;
 }>();
+
+const loading = ref(true);
+const orgStore = useOrganizationStore();
+const { organization } = storeToRefs(orgStore);
+
+onMounted(async () => {
+    await orgStore.fetchOrganization();
+    loading.value = false;
+});
 </script>
 
 <template>
@@ -26,17 +39,25 @@ defineProps<{
 
         <div>
             <div class="max-w-7xl mx-auto py-10 sm:px-6 lg:px-8">
-                <UpdateTeamNameForm :team="team" :permissions="permissions" />
+                <div v-if="loading || !organization" class="py-16 text-center text-text-secondary">
+                    Loading organization settings...
+                </div>
+                <template v-else>
+                    <UpdateTeamNameForm :team="team" :permissions="permissions" />
 
-                <SectionBorder />
-                <OrganizationBillableRate v-if="canUpdateOrganization()" :team="team" />
-                <SectionBorder />
+                    <SectionBorder />
+                    <OrganizationBillableRate v-if="canUpdateOrganization()" :team="team" />
+                    <SectionBorder />
 
-                <OrganizationFormatSettings v-if="canUpdateOrganization()" :team="team" />
-                <SectionBorder />
+                    <OrganizationFormatSettings v-if="canUpdateOrganization()" :team="team" />
+                    <SectionBorder />
 
-                <template v-if="permissions.canDeleteTeam && !team.personal_team">
-                    <DeleteTeamForm class="mt-10 sm:mt-0" :team="team" />
+                    <OrganizationTimeEntrySettings v-if="canUpdateOrganization()" />
+                    <SectionBorder />
+
+                    <template v-if="permissions.canDeleteTeam && !team.personal_team">
+                        <DeleteTeamForm class="mt-10 sm:mt-0" :team="team" />
+                    </template>
                 </template>
             </div>
         </div>
diff --git a/resources/js/packages/api/src/openapi.json.client.ts b/resources/js/packages/api/src/openapi.json.client.ts
index 5eb2bf9d..695ba6de 100644
--- a/resources/js/packages/api/src/openapi.json.client.ts
+++ b/resources/js/packages/api/src/openapi.json.client.ts
@@ -320,6 +320,7 @@ const OrganizationResource = z
         is_personal: z.boolean(),
         billable_rate: z.union([z.number(), z.null()]),
         employees_can_see_billable_rates: z.boolean(),
+        prevent_overlapping_time_entries: z.boolean(),
         currency: z.string(),
         currency_symbol: z.string(),
         number_format: NumberFormat,
@@ -334,6 +335,7 @@ const OrganizationUpdateRequest = z
         name: z.string().max(255),
         billable_rate: z.union([z.number(), z.null()]),
         employees_can_see_billable_rates: z.boolean(),
+        prevent_overlapping_time_entries: z.boolean(),
         number_format: NumberFormat,
         currency_format: CurrencyFormat,
         date_format: DateFormat,
diff --git a/tests/Unit/Endpoint/Api/V1/TimeEntryEndpointTest.php b/tests/Unit/Endpoint/Api/V1/TimeEntryEndpointTest.php
index 6e7e6cdc..585a68b6 100644
--- a/tests/Unit/Endpoint/Api/V1/TimeEntryEndpointTest.php
+++ b/tests/Unit/Endpoint/Api/V1/TimeEntryEndpointTest.php
@@ -3393,6 +3393,241 @@ class TimeEntryEndpointTest extends ApiEndpointTestAbstract
         ]);
     }
 
+    public function test_store_endpoint_blocks_overlapping_entries_when_start_overlaps(): void
+    {
+        // Arrange
+        $data = $this->createUserWithPermission([
+            'time-entries:create:own',
+        ]);
+        $data->organization->prevent_overlapping_time_entries = true;
+        $data->organization->save();
+        $baseStart = Carbon::create(2025, 1, 1, 12, 0, 0, 'UTC');
+        $baseEnd = Carbon::create(2025, 1, 1, 13, 0, 0, 'UTC');
+        TimeEntry::factory()->forOrganization($data->organization)->forMember($data->member)
+            ->create([
+                'start' => $baseStart,
+                'end' => $baseEnd,
+            ]);
+        Passport::actingAs($data->user);
+
+        // Act
+        $response = $this->postJson(route('api.v1.time-entries.store', [$data->organization->getKey()]), [
+            'member_id' => $data->member->getKey(),
+            'billable' => true,
+            'start' => $baseStart->copy()->addMinutes(30)->toIso8601ZuluString(),
+            'end' => $baseEnd->copy()->addMinutes(30)->toIso8601ZuluString(),
+        ]);
+
+        // Assert
+        $response->assertStatus(400);
+        $response->assertExactJson([
+            'error' => true,
+            'key' => 'overlapping_time_entry',
+            'message' => 'Overlapping time entries are not allowed.',
+        ]);
+    }
+
+    public function test_store_endpoint_blocks_overlapping_entries_when_end_overlaps(): void
+    {
+        // Arrange
+        $data = $this->createUserWithPermission([
+            'time-entries:create:own',
+        ]);
+        $data->organization->prevent_overlapping_time_entries = true;
+        $data->organization->save();
+        $baseStart = Carbon::create(2025, 1, 1, 12, 0, 0, 'UTC');
+        $baseEnd = Carbon::create(2025, 1, 1, 13, 0, 0, 'UTC');
+        TimeEntry::factory()->forOrganization($data->organization)->forMember($data->member)
+            ->create([
+                'start' => $baseStart,
+                'end' => $baseEnd,
+            ]);
+        Passport::actingAs($data->user);
+
+        // Act
+        $response = $this->postJson(route('api.v1.time-entries.store', [$data->organization->getKey()]), [
+            'member_id' => $data->member->getKey(),
+            'billable' => true,
+            'start' => $baseStart->copy()->subMinutes(30)->toIso8601ZuluString(),
+            'end' => $baseStart->copy()->addMinutes(30)->toIso8601ZuluString(),
+        ]);
+
+        // Assert
+        $response->assertStatus(400);
+        $response->assertExactJson([
+            'error' => true,
+            'key' => 'overlapping_time_entry',
+            'message' => 'Overlapping time entries are not allowed.',
+        ]);
+    }
+
+    public function test_store_endpoint_blocks_overlapping_entries_when_new_entry_is_within_existing(): void
+    {
+        // Arrange
+        $data = $this->createUserWithPermission([
+            'time-entries:create:own',
+        ]);
+        $data->organization->prevent_overlapping_time_entries = true;
+        $data->organization->save();
+        $baseStart = Carbon::create(2025, 1, 1, 12, 0, 0, 'UTC');
+        $baseEnd = Carbon::create(2025, 1, 1, 13, 0, 0, 'UTC');
+        TimeEntry::factory()->forOrganization($data->organization)->forMember($data->member)
+            ->create([
+                'start' => $baseStart,
+                'end' => $baseEnd,
+            ]);
+        Passport::actingAs($data->user);
+
+        // Act
+        $response = $this->postJson(route('api.v1.time-entries.store', [$data->organization->getKey()]), [
+            'member_id' => $data->member->getKey(),
+            'billable' => true,
+            'start' => $baseStart->copy()->addMinutes(15)->toIso8601ZuluString(),
+            'end' => $baseStart->copy()->addMinutes(45)->toIso8601ZuluString(),
+        ]);
+
+        // Assert
+        $response->assertStatus(400);
+        $response->assertExactJson([
+            'error' => true,
+            'key' => 'overlapping_time_entry',
+            'message' => 'Overlapping time entries are not allowed.',
+        ]);
+    }
+
+    public function test_store_endpoint_blocks_overlapping_entries_when_new_entry_surrounds_existing(): void
+    {
+        // Arrange
+        $data = $this->createUserWithPermission([
+            'time-entries:create:own',
+        ]);
+        $data->organization->prevent_overlapping_time_entries = true;
+        $data->organization->save();
+        $baseStart = Carbon::create(2025, 1, 1, 12, 0, 0, 'UTC');
+        $baseEnd = Carbon::create(2025, 1, 1, 13, 0, 0, 'UTC');
+        TimeEntry::factory()->forOrganization($data->organization)->forMember($data->member)
+            ->create([
+                'start' => $baseStart,
+                'end' => $baseEnd,
+            ]);
+        Passport::actingAs($data->user);
+
+        // Act
+        $response = $this->postJson(route('api.v1.time-entries.store', [$data->organization->getKey()]), [
+            'member_id' => $data->member->getKey(),
+            'billable' => true,
+            'start' => $baseStart->copy()->subMinutes(30)->toIso8601ZuluString(),
+            'end' => $baseEnd->copy()->addMinutes(30)->toIso8601ZuluString(),
+        ]);
+
+        // Assert
+        $response->assertStatus(400);
+        $response->assertExactJson([
+            'error' => true,
+            'key' => 'overlapping_time_entry',
+            'message' => 'Overlapping time entries are not allowed.',
+        ]);
+    }
+
+    public function test_store_endpoint_blocks_starting_active_entry_when_it_overlaps_with_existing(): void
+    {
+        // Arrange
+        $data = $this->createUserWithPermission([
+            'time-entries:create:own',
+        ]);
+        $data->organization->prevent_overlapping_time_entries = true;
+        $data->organization->save();
+        $baseStart = Carbon::create(2025, 1, 1, 12, 0, 0, 'UTC');
+        $baseEnd = Carbon::create(2025, 1, 1, 13, 0, 0, 'UTC');
+        TimeEntry::factory()->forOrganization($data->organization)->forMember($data->member)
+            ->create([
+                'start' => $baseStart,
+                'end' => $baseEnd,
+            ]);
+        Passport::actingAs($data->user);
+
+        // Act
+        $response = $this->postJson(route('api.v1.time-entries.store', [$data->organization->getKey()]), [
+            'member_id' => $data->member->getKey(),
+            'billable' => true,
+            'start' => $baseStart->copy()->addMinutes(30)->toIso8601ZuluString(),
+            'end' => null,
+        ]);
+
+        // Assert
+        $response->assertStatus(400);
+        $response->assertExactJson([
+            'error' => true,
+            'key' => 'overlapping_time_entry',
+            'message' => 'Overlapping time entries are not allowed.',
+        ]);
+    }
+
+    public function test_store_endpoint_allows_future_time_entries_even_with_running_now(): void
+    {
+        // Arrange
+        $now = Carbon::create(2025, 1, 1, 12, 0, 0, 'UTC');
+        $this->travelTo($now);
+        $data = $this->createUserWithPermission([
+            'time-entries:create:own',
+        ]);
+        $data->organization->prevent_overlapping_time_entries = true;
+        $data->organization->save();
+        TimeEntry::factory()->forOrganization($data->organization)->forMember($data->member)
+            ->create([
+                'start' => $now->copy()->subHour(),
+                'end' => null,
+            ]);
+        Passport::actingAs($data->user);
+
+        // Act
+        $response = $this->postJson(route('api.v1.time-entries.store', [$data->organization->getKey()]), [
+            'member_id' => $data->member->getKey(),
+            'billable' => true,
+            'start' => $now->copy()->addDay()->toIso8601ZuluString(),
+            'end' => $now->copy()->addDay()->addHour()->toIso8601ZuluString(),
+        ]);
+
+        // Assert
+        $response->assertStatus(201);
+    }
+
+    public function test_update_endpoint_blocks_overlap_and_excludes_current_entry(): void
+    {
+        // Arrange
+        $data = $this->createUserWithPermission([
+            'time-entries:update:own',
+        ]);
+        $data->organization->prevent_overlapping_time_entries = true;
+        $data->organization->save();
+        $baseStart = Carbon::create(2025, 1, 1, 14, 0, 0, 'UTC');
+        $baseEnd = Carbon::create(2025, 1, 1, 15, 0, 0, 'UTC');
+        $base = TimeEntry::factory()->forOrganization($data->organization)->forMember($data->member)
+            ->create([
+                'start' => $baseStart,
+                'end' => $baseEnd,
+            ]);
+        $toUpdate = TimeEntry::factory()->forOrganization($data->organization)->forMember($data->member)
+            ->create([
+                'start' => $baseEnd->copy()->addMinutes(30),
+                'end' => $baseEnd->copy()->addHour(),
+            ]);
+        Passport::actingAs($data->user);
+
+        // Act
+        $response = $this->putJson(route('api.v1.time-entries.update', [$data->organization->getKey(), $toUpdate->getKey()]), [
+            'start' => $baseStart->copy()->addMinutes(30)->toIso8601ZuluString(),
+        ]);
+
+        // Assert
+        $response->assertStatus(400);
+        $response->assertExactJson([
+            'error' => true,
+            'key' => 'overlapping_time_entry',
+            'message' => 'Overlapping time entries are not allowed.',
+        ]);
+    }
+
     public function test_update_multiple_refreshes_billable_rate_on_updates_time_entries(): void
     {
         // Arrange