diff --git a/src/Service/CalDavService.php b/src/Service/CalDavService.php
new file mode 100644
index 0000000..bcaf3a0
--- /dev/null
+++ b/src/Service/CalDavService.php
@@ -0,0 +1,353 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Service;
+
+use App\Entity\Task;
+use App\Entity\TaskRecurrence;
+use App\Enum\RecurrenceType;
+use App\Repository\ZimbraConfigurationRepository;
+use DateTimeZone;
+use Psr\Log\LoggerInterface;
+use Sabre\VObject\Component\VCalendar;
+use Symfony\Contracts\HttpClient\HttpClientInterface;
+use Throwable;
+
+final class CalDavService
+{
+    public function __construct(
+        private readonly ZimbraConfigurationRepository $configRepository,
+        private readonly TokenEncryptor $tokenEncryptor,
+        private readonly HttpClientInterface $httpClient,
+        private readonly LoggerInterface $logger,
+    ) {}
+
+    public function isConfigured(): bool
+    {
+        $config = $this->configRepository->findSingleton();
+
+        return null !== $config && $config->isEnabled();
+    }
+
+    public function testConnection(): bool
+    {
+        $config = $this->configRepository->findSingleton();
+
+        if (null === $config || !$config->isEnabled()) {
+            return false;
+        }
+
+        try {
+            $response = $this->httpClient->request('PROPFIND', $this->getCalendarUrl(), [
+                'timeout'    => 5,
+                'auth_basic' => [
+                    $config->getUsername(),
+                    $this->tokenEncryptor->decrypt((string) $config->getEncryptedPassword()),
+                ],
+                'headers' => [
+                    'Depth' => '0',
+                ],
+            ]);
+
+            $statusCode = $response->getStatusCode();
+
+            return $statusCode >= 200 && $statusCode < 300 || 207 === $statusCode;
+        } catch (Throwable $e) {
+            $this->logger->error('CalDAV connection test failed: '.$e->getMessage());
+
+            return false;
+        }
+    }
+
+    public function createEvent(Task $task): ?string
+    {
+        $uid      = $this->generateUid();
+        $calendar = $this->buildEventCalendar($task, $uid);
+
+        if (!$this->makeRequest('PUT', $this->getCalendarUrl().$uid.'.ics', $calendar->serialize())) {
+            return null;
+        }
+
+        return $uid;
+    }
+
+    public function createTodo(Task $task): ?string
+    {
+        $uid      = $this->generateUid();
+        $calendar = $this->buildTodoCalendar($task, $uid);
+
+        if (!$this->makeRequest('PUT', $this->getCalendarUrl().$uid.'.ics', $calendar->serialize())) {
+            return null;
+        }
+
+        return $uid;
+    }
+
+    public function updateEvent(Task $task): bool
+    {
+        $uid = $task->getCalendarEventUid();
+
+        if (null === $uid) {
+            return false;
+        }
+
+        $calendar = $this->buildEventCalendar($task, $uid);
+
+        return $this->makeRequest('PUT', $this->getCalendarUrl().$uid.'.ics', $calendar->serialize());
+    }
+
+    public function updateTodo(Task $task): bool
+    {
+        $uid = $task->getCalendarTodoUid();
+
+        if (null === $uid) {
+            return false;
+        }
+
+        $calendar = $this->buildTodoCalendar($task, $uid);
+
+        return $this->makeRequest('PUT', $this->getCalendarUrl().$uid.'.ics', $calendar->serialize());
+    }
+
+    public function deleteEvent(?string $uid): bool
+    {
+        if (null === $uid) {
+            return true;
+        }
+
+        return $this->makeRequest('DELETE', $this->getCalendarUrl().$uid.'.ics');
+    }
+
+    public function deleteTodo(?string $uid): bool
+    {
+        if (null === $uid) {
+            return true;
+        }
+
+        return $this->makeRequest('DELETE', $this->getCalendarUrl().$uid.'.ics');
+    }
+
+    public function syncTask(Task $task): void
+    {
+        if (!$task->isSyncToCalendar()) {
+            $this->deleteEvent($task->getCalendarEventUid());
+            $this->deleteTodo($task->getCalendarTodoUid());
+            $task->setCalendarEventUid(null);
+            $task->setCalendarTodoUid(null);
+            $task->setCalendarSyncError(null);
+
+            return;
+        }
+
+        $hasStart    = null !== $task->getScheduledStart();
+        $hasDeadline = null !== $task->getDeadline();
+
+        if (!$hasStart && !$hasDeadline) {
+            return;
+        }
+
+        $syncError = null;
+
+        if ($hasStart) {
+            if (null !== $task->getCalendarEventUid()) {
+                $success = $this->updateEvent($task);
+            } else {
+                $uid = $this->createEvent($task);
+                if (null !== $uid) {
+                    $task->setCalendarEventUid($uid);
+                    $success = true;
+                } else {
+                    $success = false;
+                }
+            }
+
+            if (!$success) {
+                $syncError = 'Failed to sync event to calendar.';
+            }
+        } elseif (null !== $task->getCalendarEventUid()) {
+            $this->deleteEvent($task->getCalendarEventUid());
+            $task->setCalendarEventUid(null);
+        }
+
+        if ($hasDeadline) {
+            if (null !== $task->getCalendarTodoUid()) {
+                $success = $this->updateTodo($task);
+            } else {
+                $uid = $this->createTodo($task);
+                if (null !== $uid) {
+                    $task->setCalendarTodoUid($uid);
+                    $success = true;
+                } else {
+                    $success = false;
+                }
+            }
+
+            if (!$success) {
+                $syncError = ($syncError ?? '').'Failed to sync todo to calendar.';
+            }
+        } elseif (null !== $task->getCalendarTodoUid()) {
+            $this->deleteTodo($task->getCalendarTodoUid());
+            $task->setCalendarTodoUid(null);
+        }
+
+        $task->setCalendarSyncError($syncError);
+    }
+
+    private function buildEventCalendar(Task $task, string $uid): VCalendar
+    {
+        $project     = $task->getProject();
+        $projectCode = null !== $project ? $project->getCode() : '';
+        $summary     = sprintf('[%s-%s] %s', $projectCode, $task->getNumber(), $task->getTitle());
+        $description = ($task->getDescription() ?? '')."\n\nLesstime task";
+
+        $vcalendar = new VCalendar();
+        $vcalendar->add('VEVENT', [
+            'UID'         => $uid,
+            'SUMMARY'     => $summary,
+            'DTSTART'     => $task->getScheduledStart(),
+            'DTEND'       => $task->getScheduledEnd(),
+            'DESCRIPTION' => $description,
+        ]);
+
+        $recurrence = $task->getRecurrence();
+
+        if (null !== $recurrence) {
+            $vevent = $vcalendar->VEVENT;
+            $vevent->add('RRULE', $this->buildRRule($recurrence));
+        }
+
+        return $vcalendar;
+    }
+
+    private function buildTodoCalendar(Task $task, string $uid): VCalendar
+    {
+        $project     = $task->getProject();
+        $projectCode = null !== $project ? $project->getCode() : '';
+        $summary     = sprintf('[%s-%s] %s (deadline)', $projectCode, $task->getNumber(), $task->getTitle());
+        $description = ($task->getDescription() ?? '')."\n\nLesstime task";
+
+        $vcalendar = new VCalendar();
+        $vcalendar->add('VTODO', [
+            'UID'         => $uid,
+            'SUMMARY'     => $summary,
+            'DUE'         => $task->getDeadline(),
+            'DESCRIPTION' => $description,
+        ]);
+
+        return $vcalendar;
+    }
+
+    private function buildRRule(TaskRecurrence $recurrence): string
+    {
+        $parts    = [];
+        $interval = $recurrence->getInterval();
+
+        match ($recurrence->getType()) {
+            RecurrenceType::Daily  => $parts[] = 'FREQ=DAILY;INTERVAL='.$interval,
+            RecurrenceType::Weekly => (function () use (&$parts, $interval, $recurrence): void {
+                $dayMap     = $this->getDayMap();
+                $daysOfWeek = $recurrence->getDaysOfWeek() ?? [];
+                $byDay      = implode(',', array_map(fn (string $d) => $dayMap[$d] ?? $d, $daysOfWeek));
+                $rule       = 'FREQ=WEEKLY;INTERVAL='.$interval;
+                if ('' !== $byDay) {
+                    $rule .= ';BYDAY='.$byDay;
+                }
+                $parts[] = $rule;
+            })(),
+            RecurrenceType::Monthly => (function () use (&$parts, $interval, $recurrence): void {
+                $dayOfMonth  = $recurrence->getDayOfMonth();
+                $weekOfMonth = $recurrence->getWeekOfMonth();
+                $daysOfWeek  = $recurrence->getDaysOfWeek() ?? [];
+
+                if (null !== $dayOfMonth) {
+                    $parts[] = 'FREQ=MONTHLY;INTERVAL='.$interval.';BYMONTHDAY='.$dayOfMonth;
+                } elseif (null !== $weekOfMonth && [] !== $daysOfWeek) {
+                    $dayMap  = $this->getDayMap();
+                    $day     = $dayMap[$daysOfWeek[0]] ?? $daysOfWeek[0];
+                    $parts[] = 'FREQ=MONTHLY;INTERVAL='.$interval.';BYDAY='.$weekOfMonth.$day;
+                } else {
+                    $parts[] = 'FREQ=MONTHLY;INTERVAL='.$interval;
+                }
+            })(),
+            RecurrenceType::Yearly => $parts[] = 'FREQ=YEARLY;INTERVAL='.$interval,
+            default                => $parts[] = 'FREQ=DAILY;INTERVAL='.$interval,
+        };
+
+        $rule = $parts[0] ?? 'FREQ=DAILY;INTERVAL=1';
+
+        $endDate        = $recurrence->getEndDate();
+        $maxOccurrences = $recurrence->getMaxOccurrences();
+
+        if (null !== $endDate) {
+            $rule .= ';UNTIL='.$endDate->setTimezone(new DateTimeZone('UTC'))->format('Ymd\THis\Z');
+        } elseif (null !== $maxOccurrences) {
+            $rule .= ';COUNT='.$maxOccurrences;
+        }
+
+        return $rule;
+    }
+
+    private function getCalendarUrl(): string
+    {
+        $config = $this->configRepository->findSingleton();
+
+        if (null === $config) {
+            return '';
+        }
+
+        return rtrim((string) $config->getServerUrl(), '/').'/'.ltrim((string) $config->getCalendarPath(), '/').'/';
+    }
+
+    private function makeRequest(string $method, string $url, ?string $body = null, string $contentType = 'text/calendar'): bool
+    {
+        $config = $this->configRepository->findSingleton();
+
+        if (null === $config) {
+            return false;
+        }
+
+        try {
+            $options = [
+                'timeout'    => 5,
+                'auth_basic' => [
+                    $config->getUsername(),
+                    $this->tokenEncryptor->decrypt((string) $config->getEncryptedPassword()),
+                ],
+            ];
+
+            if (null !== $body) {
+                $options['headers'] = ['Content-Type' => $contentType];
+                $options['body']    = $body;
+            }
+
+            $response   = $this->httpClient->request($method, $url, $options);
+            $statusCode = $response->getStatusCode();
+
+            return $statusCode >= 200 && $statusCode < 300 || 207 === $statusCode;
+        } catch (Throwable $e) {
+            $this->logger->error(sprintf('CalDAV %s request to %s failed: %s', $method, $url, $e->getMessage()));
+
+            return false;
+        }
+    }
+
+    private function generateUid(): string
+    {
+        return sprintf('%s@lesstime', bin2hex(random_bytes(16)));
+    }
+
+    /** @return array<string, string> */
+    private function getDayMap(): array
+    {
+        return [
+            'monday'    => 'MO',
+            'tuesday'   => 'TU',
+            'wednesday' => 'WE',
+            'thursday'  => 'TH',
+            'friday'    => 'FR',
+            'saturday'  => 'SA',
+            'sunday'    => 'SU',
+        ];
+    }
+}