diff --git a/app/Console/Commands/SelfHost/SelfHostDatabaseConsistency.php b/app/Console/Commands/SelfHost/SelfHostDatabaseConsistency.php
index 88a11184..5c96eebd 100644
--- a/app/Console/Commands/SelfHost/SelfHostDatabaseConsistency.php
+++ b/app/Console/Commands/SelfHost/SelfHostDatabaseConsistency.php
@@ -59,6 +59,14 @@ class SelfHostDatabaseConsistency extends Command
             ->get();
         $this->logProblems($problems, 'Time entries have a client but no project', $hadAProblem);
 
+        // End of time entries can not be before the start
+        $problems = DB::table('time_entries')
+            ->select(['id'])
+            ->whereNotNull('end')
+            ->whereColumn('end', '<', 'start')
+            ->get();
+        $this->logProblems($problems, 'Time entries have an end that is before the start', $hadAProblem);
+
         // Every user needs to be a member of at least one organization
         $problems = DB::table('users')
             ->select(['users.id as id'])
diff --git a/app/Http/Requests/V1/TimeEntry/TimeEntryUpdateRequest.php b/app/Http/Requests/V1/TimeEntry/TimeEntryUpdateRequest.php
index 92c84f32..c679d383 100644
--- a/app/Http/Requests/V1/TimeEntry/TimeEntryUpdateRequest.php
+++ b/app/Http/Requests/V1/TimeEntry/TimeEntryUpdateRequest.php
@@ -13,9 +13,11 @@ use App\Models\Tag;
 use App\Models\Task;
 use App\Models\TimeEntry;
 use App\Service\PermissionStore;
+use Carbon\Exceptions\InvalidFormatException;
 use Closure;
 use Illuminate\Contracts\Validation\ValidationRule;
 use Illuminate\Database\Eloquent\Builder;
+use Illuminate\Support\Carbon;
 use Illuminate\Support\Facades\Auth;
 use Illuminate\Validation\ConditionalRules;
 use Illuminate\Validation\Rule;
@@ -90,12 +92,32 @@ class TimeEntryUpdateRequest extends BaseFormRequest
             // Start of time entry (Format: "Y-m-d\TH:i:s\Z", UTC timezone, Example: "2000-02-22T14:58:59Z")
             'start' => [
                 'date_format:Y-m-d\TH:i:s\Z',
+                function (string $attribute, mixed $value, Closure $fail) use ($timeEntry): void {
+                    // If the payload does not contain an end, the start needs to be validated against the persisted end
+                    if ($this->has('end') || $timeEntry?->end === null) {
+                        return;
+                    }
+                    $start = $this->parseDate($value);
+                    if ($start !== null && $start->gt($timeEntry->end)) {
+                        $fail('The start field must be a date before or equal to end.');
+                    }
+                },
             ],
             // End of time entry (Format: "Y-m-d\TH:i:s\Z", UTC timezone, Example: "2000-02-22T14:58:59Z")
             'end' => [
                 'nullable',
                 'date_format:Y-m-d\TH:i:s\Z',
                 'after_or_equal:start',
+                function (string $attribute, mixed $value, Closure $fail) use ($timeEntry): void {
+                    // If the payload does not contain a start, the end needs to be validated against the persisted start
+                    if ($this->has('start') || $timeEntry === null) {
+                        return;
+                    }
+                    $end = $this->parseDate($value);
+                    if ($end !== null && $end->lt($timeEntry->start)) {
+                        $fail('The end field must be a date after or equal to start.');
+                    }
+                },
             ],
             // Whether time entry is billable
             'billable' => [
@@ -137,4 +159,16 @@ class TimeEntryUpdateRequest extends BaseFormRequest
             ],
         ];
     }
+
+    private function parseDate(mixed $value): ?Carbon
+    {
+        if (! is_string($value)) {
+            return null;
+        }
+        try {
+            return Carbon::createFromFormat('Y-m-d\TH:i:s\Z', $value, 'UTC');
+        } catch (InvalidFormatException) {
+            return null;
+        }
+    }
 }
diff --git a/app/Service/Import/Importers/ClockifyTimeEntriesImporter.php b/app/Service/Import/Importers/ClockifyTimeEntriesImporter.php
index f74919b2..7ec2d2be 100644
--- a/app/Service/Import/Importers/ClockifyTimeEntriesImporter.php
+++ b/app/Service/Import/Importers/ClockifyTimeEntriesImporter.php
@@ -190,6 +190,9 @@ class ClockifyTimeEntriesImporter extends DefaultImporter
                 if ($end === null) {
                     throw new ImportException('End date ("'.$endDateStr.'") or time ("'.$endTimeStr.'") are invalid');
                 }
+                if ($end->lt($start)) {
+                    throw new ImportException('End ("'.$endStr.'") is before start ("'.$startStr.'")');
+                }
                 $timeEntry->end = $end->utc();
 
                 $timeEntry->billable_rate = $this->billableRateService->getBillableRateForTimeEntryWithGivenRelations(
diff --git a/app/Service/Import/Importers/GenericTimeEntriesImporter.php b/app/Service/Import/Importers/GenericTimeEntriesImporter.php
index bb19ac0e..c0fe90b7 100644
--- a/app/Service/Import/Importers/GenericTimeEntriesImporter.php
+++ b/app/Service/Import/Importers/GenericTimeEntriesImporter.php
@@ -155,6 +155,9 @@ class GenericTimeEntriesImporter extends DefaultImporter
                 if ($end === null) {
                     throw new ImportException('Value of end ("'.$record['end'].'") is invalid');
                 }
+                if ($end->lt($start)) {
+                    throw new ImportException('Value of end ("'.$record['end'].'") is before start ("'.$record['start'].'")');
+                }
                 $timeEntry->end = $end->utc();
                 $timeEntry->billable_rate = $this->billableRateService->getBillableRateForTimeEntryWithGivenRelations(
                     $timeEntry,
diff --git a/app/Service/Import/Importers/HarvestTimeEntriesImporter.php b/app/Service/Import/Importers/HarvestTimeEntriesImporter.php
index 57a66f49..77e965ef 100644
--- a/app/Service/Import/Importers/HarvestTimeEntriesImporter.php
+++ b/app/Service/Import/Importers/HarvestTimeEntriesImporter.php
@@ -136,6 +136,9 @@ class HarvestTimeEntriesImporter extends DefaultImporter
                     throw new ImportException('Hours ("'.$record['Hours'].'") is invalid');
                 }
                 $hours = (float) $hoursField;
+                if ($hours < 0) {
+                    throw new ImportException('Hours ("'.$record['Hours'].'") is negative');
+                }
                 $timeEntry->start = $date->copy()->startOfDay()->utc();
                 $timeEntry->end = $date->copy()->startOfDay()->addHours($hours)->utc();
                 $timeEntry->billable_rate = $this->billableRateService->getBillableRateForTimeEntryWithGivenRelations(
diff --git a/app/Service/Import/Importers/SolidtimeImporter.php b/app/Service/Import/Importers/SolidtimeImporter.php
index f6eade0e..7089d469 100644
--- a/app/Service/Import/Importers/SolidtimeImporter.php
+++ b/app/Service/Import/Importers/SolidtimeImporter.php
@@ -280,6 +280,9 @@ class SolidtimeImporter extends DefaultImporter
                     if ($end === null) {
                         throw new ImportException('End date ("'.$timeEntryRow['end'].'") is invalid');
                     }
+                    if ($end->lt($start)) {
+                        throw new ImportException('End date ("'.$timeEntryRow['end'].'") is before start date ("'.$timeEntryRow['start'].'")');
+                    }
                     $timeEntry->end = $end->utc();
                 } else {
                     $timeEntry->end = null;
diff --git a/app/Service/Import/Importers/TogglTimeEntriesImporter.php b/app/Service/Import/Importers/TogglTimeEntriesImporter.php
index e18cf606..c2f70843 100644
--- a/app/Service/Import/Importers/TogglTimeEntriesImporter.php
+++ b/app/Service/Import/Importers/TogglTimeEntriesImporter.php
@@ -139,6 +139,9 @@ class TogglTimeEntriesImporter extends DefaultImporter
                 if ($end === null) {
                     throw new ImportException('End date ("'.$record['End date'].'") or time ("'.$record['End time'].'") are invalid');
                 }
+                if ($end->lt($start)) {
+                    throw new ImportException('End ("'.$record['End date'].' '.$record['End time'].'") is before start ("'.$record['Start date'].' '.$record['Start time'].'")');
+                }
                 $timeEntry->end = $end->utc();
                 $timeEntry->billable_rate = $this->billableRateService->getBillableRateForTimeEntryWithGivenRelations(
                     $timeEntry,
diff --git a/tests/Unit/Console/Commands/SelfHost/SelfHostDatabaseConsistencyCommandTest.php b/tests/Unit/Console/Commands/SelfHost/SelfHostDatabaseConsistencyCommandTest.php
index 26491860..b235d38a 100644
--- a/tests/Unit/Console/Commands/SelfHost/SelfHostDatabaseConsistencyCommandTest.php
+++ b/tests/Unit/Console/Commands/SelfHost/SelfHostDatabaseConsistencyCommandTest.php
@@ -13,6 +13,7 @@ use App\Models\Task;
 use App\Models\TimeEntry;
 use App\Models\User;
 use Illuminate\Console\Command;
+use Illuminate\Support\Carbon;
 use Illuminate\Support\Facades\Artisan;
 use PHPUnit\Framework\Attributes\CoversClass;
 use Tests\TestCaseWithDatabase;
@@ -158,4 +159,26 @@ class SelfHostDatabaseConsistencyCommandTest extends TestCaseWithDatabase
         $output = Artisan::output();
         $this->assertSame("Consistency problem: Users have a current organization that they are not a member of\n  - ".$user1->user->getKey()."\n", $output);
     }
+
+    public function test_checks_that_end_of_time_entries_is_not_before_start(): void
+    {
+        // Arrange
+        $user = $this->createUserWithRole(Role::Owner);
+        $timeEntry = TimeEntry::factory()->forMember($user->member)->create([
+            'start' => Carbon::parse('2026-08-01T03:00:00Z'),
+            'end' => Carbon::parse('2026-08-01T02:59:55Z'),
+        ]);
+        TimeEntry::factory()->forMember($user->member)->create([
+            'start' => Carbon::parse('2026-08-01T04:00:00Z'),
+            'end' => Carbon::parse('2026-08-01T04:00:00Z'),
+        ]);
+
+        // Act
+        $exitCode = $this->withoutMockingConsoleOutput()->artisan('self-host:database-consistency');
+
+        // Assert
+        $this->assertSame(Command::FAILURE, $exitCode);
+        $output = Artisan::output();
+        $this->assertSame("Consistency problem: Time entries have an end that is before the start\n  - ".$timeEntry->getKey()."\n", $output);
+    }
 }
diff --git a/tests/Unit/Endpoint/Api/V1/TimeEntryEndpointTest.php b/tests/Unit/Endpoint/Api/V1/TimeEntryEndpointTest.php
index 733693cb..fb32eaff 100644
--- a/tests/Unit/Endpoint/Api/V1/TimeEntryEndpointTest.php
+++ b/tests/Unit/Endpoint/Api/V1/TimeEntryEndpointTest.php
@@ -2961,6 +2961,101 @@ class TimeEntryEndpointTest extends ApiEndpointTestAbstract
         ]);
     }
 
+    public function test_update_endpoint_validation_fails_if_only_end_is_sent_and_it_is_before_the_persisted_start(): void
+    {
+        // Arrange
+        $data = $this->createUserWithPermission([
+            'time-entries:update:own',
+        ]);
+        $timeEntry = TimeEntry::factory()->forOrganization($data->organization)->forMember($data->member)->create([
+            'start' => Carbon::parse('2026-08-01T03:00:00Z'),
+            'end' => Carbon::parse('2026-08-01T03:01:00Z'),
+        ]);
+        Passport::actingAs($data->user);
+
+        // Act
+        $response = $this->putJson(route('api.v1.time-entries.update', [$data->organization->getKey(), $timeEntry->getKey()]), [
+            'end' => '2026-08-01T02:59:55Z',
+        ]);
+
+        // Assert
+        $response->assertStatus(422);
+        $response->assertJsonValidationErrors([
+            'end' => 'The end field must be a date after or equal to start.',
+        ]);
+        $timeEntry->refresh();
+        $this->assertSame('2026-08-01T03:01:00Z', $timeEntry->end->toIso8601ZuluString());
+    }
+
+    public function test_update_endpoint_validation_fails_if_only_start_is_sent_and_it_is_after_the_persisted_end(): void
+    {
+        // Arrange
+        $data = $this->createUserWithPermission([
+            'time-entries:update:own',
+        ]);
+        $timeEntry = TimeEntry::factory()->forOrganization($data->organization)->forMember($data->member)->create([
+            'start' => Carbon::parse('2026-08-01T03:00:00Z'),
+            'end' => Carbon::parse('2026-08-01T03:01:00Z'),
+        ]);
+        Passport::actingAs($data->user);
+
+        // Act
+        $response = $this->putJson(route('api.v1.time-entries.update', [$data->organization->getKey(), $timeEntry->getKey()]), [
+            'start' => '2026-08-01T03:01:05Z',
+        ]);
+
+        // Assert
+        $response->assertStatus(422);
+        $response->assertJsonValidationErrors([
+            'start' => 'The start field must be a date before or equal to end.',
+        ]);
+        $timeEntry->refresh();
+        $this->assertSame('2026-08-01T03:00:00Z', $timeEntry->start->toIso8601ZuluString());
+    }
+
+    public function test_update_endpoint_allows_updating_only_start_of_running_time_entry(): void
+    {
+        // Arrange
+        $data = $this->createUserWithPermission([
+            'time-entries:update:own',
+        ]);
+        $timeEntry = TimeEntry::factory()->forOrganization($data->organization)->forMember($data->member)->active()->create();
+        Passport::actingAs($data->user);
+
+        // Act
+        $response = $this->putJson(route('api.v1.time-entries.update', [$data->organization->getKey(), $timeEntry->getKey()]), [
+            'start' => '2026-08-01T03:00:00Z',
+        ]);
+
+        // Assert
+        $response->assertStatus(200);
+        $timeEntry->refresh();
+        $this->assertSame('2026-08-01T03:00:00Z', $timeEntry->start->toIso8601ZuluString());
+    }
+
+    public function test_update_endpoint_allows_updating_only_end_if_it_is_after_the_persisted_start(): void
+    {
+        // Arrange
+        $data = $this->createUserWithPermission([
+            'time-entries:update:own',
+        ]);
+        $timeEntry = TimeEntry::factory()->forOrganization($data->organization)->forMember($data->member)->create([
+            'start' => Carbon::parse('2026-08-01T03:00:00Z'),
+            'end' => Carbon::parse('2026-08-01T03:01:00Z'),
+        ]);
+        Passport::actingAs($data->user);
+
+        // Act
+        $response = $this->putJson(route('api.v1.time-entries.update', [$data->organization->getKey(), $timeEntry->getKey()]), [
+            'end' => '2026-08-01T03:00:00Z',
+        ]);
+
+        // Assert
+        $response->assertStatus(200);
+        $timeEntry->refresh();
+        $this->assertSame('2026-08-01T03:00:00Z', $timeEntry->end->toIso8601ZuluString());
+    }
+
     public function test_update_endpoint_validation_fails_if_project_id_is_missing_but_request_has_task_id(): void
     {
         // Arrange
diff --git a/tests/Unit/Service/Import/Importers/ClockifyTimeEntriesImporterTest.php b/tests/Unit/Service/Import/Importers/ClockifyTimeEntriesImporterTest.php
index 22d6d693..15be2f52 100644
--- a/tests/Unit/Service/Import/Importers/ClockifyTimeEntriesImporterTest.php
+++ b/tests/Unit/Service/Import/Importers/ClockifyTimeEntriesImporterTest.php
@@ -231,4 +231,29 @@ class ClockifyTimeEntriesImporterTest extends ImporterTestAbstract
         $this->assertSame(0, Tag::query()->count());
         $this->assertSame(0, Client::query()->count());
     }
+
+    public function test_import_fails_if_end_is_before_start(): void
+    {
+        // Arrange
+        $organization = Organization::factory()->create();
+        $timezone = 'Europe/Vienna';
+        $importer = new ClockifyTimeEntriesImporter;
+        $importer->init($organization);
+        $data = <<<'CSV'
+        "Project","Client","Description","Task","User","Group","Email","Tags","Billable","Start Date","Start Time","End Date","End Time","Duration (h)","Duration (decimal)","Billable Rate (USD)","Billable Amount (USD)"
+        "Project","Client","Working hard","","Peter Tester","","peter.test@email.test","","Yes","03/04/2024","10:30:00 AM","03/04/2024","10:00:00 AM","00:30:00","0.50","0.00","0.00"
+        CSV;
+
+        // Act
+        try {
+            $importer->importData($data, $timezone);
+        } catch (ImportException $e) {
+            // Assert
+            $this->assertSame('End ("03/04/2024 10:00:00 AM") is before start ("03/04/2024 10:30:00 AM")', $e->getMessage());
+            $this->assertSame(0, TimeEntry::query()->count());
+
+            return;
+        }
+        $this->fail();
+    }
 }
diff --git a/tests/Unit/Service/Import/Importers/GenericTimeEntriesImporterTest.php b/tests/Unit/Service/Import/Importers/GenericTimeEntriesImporterTest.php
index 69db995c..fb9e76fd 100644
--- a/tests/Unit/Service/Import/Importers/GenericTimeEntriesImporterTest.php
+++ b/tests/Unit/Service/Import/Importers/GenericTimeEntriesImporterTest.php
@@ -5,6 +5,7 @@ declare(strict_types=1);
 namespace Tests\Unit\Service\Import\Importers;
 
 use App\Models\Organization;
+use App\Models\TimeEntry;
 use App\Service\Import\Importers\DefaultImporter;
 use App\Service\Import\Importers\GenericTimeEntriesImporter;
 use App\Service\Import\Importers\ImportException;
@@ -89,4 +90,27 @@ class GenericTimeEntriesImporterTest extends ImporterTestAbstract
         }
         $this->fail();
     }
+
+    public function test_import_fails_if_end_is_before_start(): void
+    {
+        // Arrange
+        $organization = Organization::factory()->create();
+        $timezone = 'Europe/Vienna';
+        $importer = new GenericTimeEntriesImporter;
+        $importer->init($organization);
+        $data = "description,billable,client,project,tags,start,end,task,user_name,user_email\n".
+            '"Working hard","true","Big Company","Project for Big Company","","2024-03-04T10:23:00Z","2024-03-04T09:23:00Z","","Peter Tester","peter.test@email.test"';
+
+        // Act
+        try {
+            $importer->importData($data, $timezone);
+        } catch (ImportException $e) {
+            // Assert
+            $this->assertSame('Value of end ("2024-03-04T09:23:00Z") is before start ("2024-03-04T10:23:00Z")', $e->getMessage());
+            $this->assertSame(0, TimeEntry::query()->count());
+
+            return;
+        }
+        $this->fail();
+    }
 }
diff --git a/tests/Unit/Service/Import/Importers/HarvestTimeEntriesImporterTest.php b/tests/Unit/Service/Import/Importers/HarvestTimeEntriesImporterTest.php
index c94f93c5..81f986b1 100644
--- a/tests/Unit/Service/Import/Importers/HarvestTimeEntriesImporterTest.php
+++ b/tests/Unit/Service/Import/Importers/HarvestTimeEntriesImporterTest.php
@@ -105,4 +105,27 @@ class HarvestTimeEntriesImporterTest extends ImporterTestAbstract
         $this->assertSame(2, $report->projectsCreated);
         $this->assertSame(1, $report->clientsCreated);
     }
+
+    public function test_import_fails_if_hours_are_negative(): void
+    {
+        // Arrange
+        $organization = Organization::factory()->create();
+        $timezone = 'Europe/Vienna';
+        $importer = new HarvestTimeEntriesImporter;
+        $importer->init($organization);
+        $data = "Date,Client,Project,Project Code,Task,Notes,Hours,Billable?,Invoiced?,Approved?,First Name,Last Name,Roles,Employee?,Billable Rate,Billable Amount,Cost Rate,Cost Amount,Currency,External Reference URL\n".
+            '2024-03-04,,Project without Client,,,"","-2,0",No,No,No,Peter,Tester,,Yes,"100,0","2.000,0","0,0","0,0",Euro - EUR,';
+
+        // Act
+        try {
+            $importer->importData($data, $timezone);
+        } catch (ImportException $e) {
+            // Assert
+            $this->assertSame('Hours ("-2,0") is negative', $e->getMessage());
+            $this->assertSame(0, TimeEntry::query()->count());
+
+            return;
+        }
+        $this->fail();
+    }
 }
diff --git a/tests/Unit/Service/Import/Importers/SolidtimeImporterTest.php b/tests/Unit/Service/Import/Importers/SolidtimeImporterTest.php
index 3e93d675..dc390dc6 100644
--- a/tests/Unit/Service/Import/Importers/SolidtimeImporterTest.php
+++ b/tests/Unit/Service/Import/Importers/SolidtimeImporterTest.php
@@ -15,7 +15,9 @@ use App\Service\Import\Importers\SolidtimeImporter;
 use Exception;
 use Illuminate\Support\Facades\DB;
 use Illuminate\Support\Facades\Queue;
+use Illuminate\Support\Str;
 use PHPUnit\Framework\Attributes\CoversClass;
+use ZipArchive;
 
 #[CoversClass(SolidtimeImporter::class)]
 #[CoversClass(ImportException::class)]
@@ -177,4 +179,32 @@ class SolidtimeImporterTest extends ImporterTestAbstract
         Queue::assertPushed(RecalculateSpentTimeForProject::class, 1);
         Queue::assertPushed(RecalculateSpentTimeForTask::class, 1);
     }
+
+    public function test_import_fails_if_end_of_time_entry_is_before_start(): void
+    {
+        // Arrange
+        $organization = Organization::factory()->create();
+        $timezone = 'Europe/Vienna';
+        $zipPath = $this->createTestZip('solidtime_import_test_1');
+        $zip = new ZipArchive;
+        $zip->open($zipPath);
+        $timeEntries = $zip->getFromName('time_entries.csv');
+        $timeEntries = Str::replaceFirst(',2024-03-04T09:23:52Z,2024-03-04T09:23:52Z,', ',2024-03-04T09:23:52Z,2024-03-04T08:23:52Z,', $timeEntries);
+        $zip->addFromString('time_entries.csv', $timeEntries);
+        $zip->close();
+        $importer = new SolidtimeImporter;
+        $importer->init($organization);
+        $data = file_get_contents($zipPath);
+
+        // Act
+        try {
+            $importer->importData($data, $timezone);
+        } catch (ImportException $e) {
+            // Assert
+            $this->assertSame('End date ("2024-03-04T08:23:52Z") is before start date ("2024-03-04T09:23:52Z")', $e->getMessage());
+
+            return;
+        }
+        $this->fail();
+    }
 }
diff --git a/tests/Unit/Service/Import/Importers/TogglTimeEntriesImporterTest.php b/tests/Unit/Service/Import/Importers/TogglTimeEntriesImporterTest.php
index 719bea69..61e9f11a 100644
--- a/tests/Unit/Service/Import/Importers/TogglTimeEntriesImporterTest.php
+++ b/tests/Unit/Service/Import/Importers/TogglTimeEntriesImporterTest.php
@@ -120,4 +120,27 @@ class TogglTimeEntriesImporterTest extends ImporterTestAbstract
         Queue::assertPushed(RecalculateSpentTimeForProject::class, 2);
         Queue::assertPushed(RecalculateSpentTimeForTask::class, 1);
     }
+
+    public function test_import_fails_if_end_is_before_start(): void
+    {
+        // Arrange
+        $organization = Organization::factory()->create();
+        $timezone = 'Europe/Vienna';
+        $importer = new TogglTimeEntriesImporter;
+        $importer->init($organization);
+        $data = "User,Email,Client,Project,Task,Description,Billable,Start date,Start time,End date,End time,Duration,Tags,Amount (EUR)\n".
+            'Peter Tester,peter.test@email.test,,Project without Client,,"",No,2024-03-04,10:23:52,2024-03-04,09:23:52,-01:00:00,"",';
+
+        // Act
+        try {
+            $importer->importData($data, $timezone);
+        } catch (ImportException $e) {
+            // Assert
+            $this->assertSame('End ("2024-03-04 09:23:52") is before start ("2024-03-04 10:23:52")', $e->getMessage());
+            $this->assertSame(0, TimeEntry::query()->count());
+
+            return;
+        }
+        $this->fail();
+    }
 }