diff --git a/lib/page/classtable/classtable_state.dart b/lib/page/classtable/classtable_state.dart
index 68ec599a..7be01c0c 100644
--- a/lib/page/classtable/classtable_state.dart
+++ b/lib/page/classtable/classtable_state.dart
@@ -6,21 +6,19 @@ import 'dart:math' as math;
 
 import 'package:device_calendar/device_calendar.dart';
 import 'package:flutter/material.dart';
-import 'package:intl/intl.dart';
 import 'package:signals/signals.dart';
-import 'package:timezone/data/latest.dart' as tz;
 
 import 'package:watermeter/controller/classtable_controller.dart';
 import 'package:watermeter/controller/exam_controller.dart';
-import 'package:watermeter/controller/physics_experiment_controller.dart';
 import 'package:watermeter/controller/other_experiment_controller.dart';
+import 'package:watermeter/controller/physics_experiment_controller.dart';
 import 'package:watermeter/controller/week_swift_controller.dart';
-import 'package:watermeter/model/time_list.dart';
 import 'package:watermeter/model/xidian_ids/classtable.dart';
 import 'package:watermeter/model/xidian_ids/exam.dart';
 import 'package:watermeter/model/xidian_ids/experiment.dart';
 import 'package:watermeter/page/classtable/class_table_view/class_organized_data.dart';
 import 'package:watermeter/repository/logger.dart';
+import 'package:watermeter/repository/system_calendar_sync_service.dart';
 import 'package:watermeter/themes/color_seed.dart';
 
 /// Use a inheritedWidget to share the ClassTableWidgetState
@@ -304,285 +302,35 @@ class ClassTableWidgetState with ChangeNotifier {
           .deleteUserDefinedClass(timeArrangement)
           .then((value) => notifyListeners());
 
-  List<Event> get events {
-    List<Event> events = [];
-
-    // UTC+8 timezone defination, hard-coded since our school is in here...
-    tz.initializeTimeZones();
-    Location currentLocation = getLocation("Asia/Shanghai");
-
-    // @hgh: i here means each single course assignment
-    for (var i in timeArrangement) {
-      // @hgh: j here means each week for a single course assignment
-      // @hgh: find the first week that has class
-      int j = i.weekList.indexWhere((element) => element);
-
-      // @benderblog: basically not happens, but if not arranged, just skip it.
-      if (j == -1) continue;
-
-      // @benderblog: rewrite, using ai generated algorithm to get the ranges of "true".
-      List<(int, int)> ranges = [];
-      int start = -1;
-
-      for (j; j < i.weekList.length; j++) {
-        if (i.weekList[j] && start == -1) {
-          start = j;
-        } else if (!i.weekList[j] && start != -1) {
-          ranges.add((start, j - 1));
-          start = -1;
-        }
-      }
-
-      // @ai: Handle the case where the array ends with a sequence of true values.
-      if (start != -1) {
-        ranges.add((start, j - 1));
-      }
-
-      String title =
-          "${getClassDetail(timeArrangement.indexOf(i)).name}@${i.classroom ?? "待定"}";
-      String description =
-          "课程名称：${getClassDetail(timeArrangement.indexOf(i)).name} - 老师：${i.teacher ?? "未知"}";
-      String? location = i.classroom ?? "待定";
-
-      List<String> startTime = timeList[(i.start - 1) * 2].split(":");
-      List<String> stopTime = timeList[(i.stop - 1) * 2 + 1].split(":");
-
-      DayOfWeek getDayOfWeek(int day) {
-        switch (day) {
-          case 1:
-            return DayOfWeek.Monday;
-          case 2:
-            return DayOfWeek.Tuesday;
-          case 3:
-            return DayOfWeek.Wednesday;
-          case 4:
-            return DayOfWeek.Thursday;
-          case 5:
-            return DayOfWeek.Friday;
-          case 6:
-            return DayOfWeek.Saturday;
-          case 7:
-            return DayOfWeek.Sunday;
-          default:
-            return DayOfWeek.Sunday;
-        }
-      }
-
-      // @benderblog: start dealing with
-      for (var range in ranges) {
-        // @hgh: initialize the first day(or, first recurrence) of the class
-        DateTime firstDay = startDay.add(
-          Duration(days: range.$1 * 7 + i.day - 1),
-        );
-
-        DateTime startTimeToUse = firstDay.add(
-          Duration(
-            hours: int.parse(startTime[0]),
-            minutes: int.parse(startTime[1]),
-          ),
-        );
-        DateTime stopTimeToUse = firstDay.add(
-          Duration(
-            hours: int.parse(stopTime[0]),
-            minutes: int.parse(stopTime[1]),
-          ),
-        );
-
-        RecurrenceRule rrule = RecurrenceRule(
-          RecurrenceFrequency.Weekly,
-          daysOfWeek: [getDayOfWeek(i.day)],
-          endDate: firstDay.add(Duration(days: (range.$2 - range.$1) * 7 + 1)),
-        );
-
-        events.add(
-          Event(
-            null,
-            title: title,
-            description: description,
-            recurrenceRule: rrule,
-            start: TZDateTime.from(startTimeToUse, currentLocation),
-            end: TZDateTime.from(stopTimeToUse, currentLocation),
-            location: location,
-          ),
-        );
-      }
-    }
-
-    // Add exam data and experiment data here.
-    for (var i in subjects) {
-      String title =
-          "${i.subject} ${i.typeStr}@${i.place} ${i.seat != null ? "-${i.seat}" : ""}";
-      String description = "考试信息：${i.subject} - ${i.typeStr}";
-      String location = "${i.place} ${i.seat != null ? "-${i.seat}" : ""}";
-
-      events.add(
-        Event(
-          null,
-          title: title,
-          description: description,
-          start: TZDateTime.from(i.startTime!, currentLocation),
-          end: TZDateTime.from(i.stopTime!, currentLocation),
-          location: location,
-        ),
-      );
-    }
-
-    for (var experiment in experiments) {
-      for (var j in experiment.timeRanges) {
-        events.add(
-          Event(
-            null,
-            title: "${experiment.name}@${experiment.classroom}",
-            description: "实验名称：${experiment.name} - 老师：${experiment.teacher}",
-            start: TZDateTime.from(j.$1, currentLocation),
-            end: TZDateTime.from(j.$2, currentLocation),
-            location: experiment.classroom,
-          ),
-        );
-      }
-    }
-    return events;
-  }
+  List<Event> get events => buildCalendarEvents(
+    classTableData: classTableController.classTableComputedSignal.value,
+    subjects: subjects,
+    experiments: experiments,
+  );
 
   /// Generate icalendar file string.
-  String get iCalenderStr {
-    String toReturn = '''BEGIN:VCALENDAR
-CALSCALE:GREGORIAN
-BEGIN:VTIMEZONE
-TZID:Asia/Shanghai
-X-LIC-LOCATION:Asia/Shanghai
-BEGIN:STANDARD
-TZOFFSETFROM:+0800
-TZOFFSETTO:+0800
-TZNAME:CST
-DTSTART:19700101T000000
-END:STANDARD
-END:VTIMEZONE
-''';
-
-    for (var i in events) {
-      String vevent = "BEGIN:VEVENT\n";
-
-      vevent +=
-          "DTSTAMP:"
-          "${DateFormat('yyyyMMddTHHmmssZ').format(DateTime.now())}\n";
-      vevent += "SUMMARY:${i.title ?? "待定"}\n";
-      vevent += "DESCRIPTION:${i.description ?? "待定"}\n";
-
-      /// Minus 8 hours to match the "UTC time"
-      vevent +=
-          "DTSTART;TZID=Asia/Shanghai:"
-          "${DateFormat('yyyyMMddTHHmmss').format(DateTime.fromMicrosecondsSinceEpoch(i.start!.microsecondsSinceEpoch))}\n";
-      vevent +=
-          "DTEND;TZID=Asia/Shanghai:"
-          "${DateFormat('yyyyMMddTHHmmss').format(DateTime.fromMicrosecondsSinceEpoch(i.end!.microsecondsSinceEpoch))}\n";
-      if (i.location != null) {
-        vevent += "LOCATION:${i.location}\n";
-      }
-
-      if (i.recurrenceRule != null) {
-        String getWeekStr(DayOfWeek day) {
-          switch (day) {
-            case DayOfWeek.Monday:
-              return "MO";
-            case DayOfWeek.Tuesday:
-              return "TU";
-            case DayOfWeek.Wednesday:
-              return "WE";
-            case DayOfWeek.Thursday:
-              return "TH";
-            case DayOfWeek.Friday:
-              return "FR";
-            case DayOfWeek.Saturday:
-              return "SA";
-            case DayOfWeek.Sunday:
-              return "SU";
-          }
-        }
-
-        vevent +=
-            "RRULE:FREQ=WEEKLY;INTERVAL=1;BYDAY=${getWeekStr(i.recurrenceRule!.daysOfWeek!.first)};"
-            "UNTIL=${DateFormat('yyyyMMdd').format(i.recurrenceRule!.endDate!)}\n";
-      }
-      toReturn += "${vevent}END:VEVENT\n";
-    }
-
-    return "${toReturn}END:VCALENDAR";
-  }
+  String get iCalenderStr => buildICalendarString(events);
 
   /// Output to System Calendar
   Future<bool> outputToCalendar(Future<void> Function() showDialog) async {
-    // Fetch calendar permission
-    final DeviceCalendarPlugin deviceCalendarPlugin = DeviceCalendarPlugin();
-    Result<bool> hasPermitted = await deviceCalendarPlugin.hasPermissions();
-    if (hasPermitted.data != true) {
-      await showDialog();
-      hasPermitted = await deviceCalendarPlugin.requestPermissions();
-      if (hasPermitted.data != true) {
-        log.info(
-          "[Classtable][outputToCalendar] "
-          "Gain permission failed: "
-          "${hasPermitted.errors.map((e) => e.errorMessage).join(",")}",
-        );
-        return false;
-      }
-    }
-
-    // Generate a new calendar
-    (bool, String) calendarIdData = await DeviceCalendarPlugin()
-        .createCalendar(
-          "PDA Class Arrangement $semesterCode "
-          "created at ${DateTime.now().millisecondsSinceEpoch}",
-        )
-        .then((data) {
-          if (!data.isSuccess) {
-            log.info(
-              "[Classtable][outputToCalendar] "
-              "Generate new calendar failed: "
-              "${hasPermitted.errors.map((e) => e.errorMessage).join(",")}",
-            );
-            return (false, "");
-          } else {
-            return (true, data.data!);
-          }
-        });
-
-    if (!calendarIdData.$1) {
-      return false;
-    }
-    String calendarId = calendarIdData.$2;
-
-    for (var i in events) {
-      var toPush = i..calendarId = calendarId;
-      Result<String>? addEventResult = await deviceCalendarPlugin
-          .createOrUpdateEvent(toPush);
-      // If got error, return with false.
-      if (addEventResult == null ||
-          addEventResult.data == null ||
-          addEventResult.data!.isEmpty) {
-        log.info(
-          "[Classtable][outputToCalendar] "
-          "Add failed: "
-          "${hasPermitted.errors.map((e) => e.errorMessage).join(",")}",
-        );
-        return false;
-      }
-    }
-
-    return true;
+    return await SystemCalendarSyncService().syncSystemCalendar(
+      requestPermissionsIfNeeded: true,
+      onlyIfCalendarExists: false,
+      showDialog: showDialog,
+    );
   }
 
   /// Update classtable infos
   Future<void> updateClasstable(BuildContext context) async {
     log.info("Updating time arrangement data...");
-    return await Future.wait([
+    await Future.wait([
       classTableController.reloadClassTable(),
       examController.reloadExamInfo(),
       physicsExperimentController.reloadPhysicsExperiment(),
       otherExperimentController.reloadOtherExperiment(),
-    ]).then((value) {
-      notifyListeners();
-    });
+    ]);
+    await maybeAutoSyncSystemCalendar();
+    notifyListeners();
   }
 
   ClassTableWidgetState({required this.currentWeek}) {
diff --git a/lib/page/homepage/refresh.dart b/lib/page/homepage/refresh.dart
index d5a41b33..79d82102 100644
--- a/lib/page/homepage/refresh.dart
+++ b/lib/page/homepage/refresh.dart
@@ -15,6 +15,7 @@ import 'package:watermeter/controller/schoolnet_controller.dart';
 import 'package:watermeter/controller/semester_controller.dart';
 import 'package:watermeter/repository/logger.dart';
 import 'package:watermeter/repository/notification/course_reminder_service.dart';
+import 'package:watermeter/repository/system_calendar_sync_service.dart';
 import 'package:watermeter/repository/xidian_ids/ids_session.dart';
 
 Future<void> _comboLogin({
@@ -81,9 +82,13 @@ Future<void> update({
     ),
     _safeReload("Library", LibraryController.i.reloadBorrowList),
     _safeReload("SchoolCard", SchoolCardController.i.reloadOverview),
-    _safeReload("Electricity", ElectricityController.i.refreshElectricityInfo),
+    _safeReload(
+      "Electricity",
+      ElectricityController.i.refreshElectricityInfo,
+    ),
     _safeReload("Schoolnet", SchoolnetController.i.reloadSchoolnetInfo),
   ]);
+  await maybeAutoSyncSystemCalendar();
 
   if (CourseReminderService().isInitialized) {
     CourseReminderService().validateAndUpdateNotifications();
diff --git a/lib/page/setting/setting.dart b/lib/page/setting/setting.dart
index c523b6cf..6a5dc350 100644
--- a/lib/page/setting/setting.dart
+++ b/lib/page/setting/setting.dart
@@ -4,6 +4,7 @@
 
 // Setting window.
 
+import 'dart:async';
 import 'dart:io';
 
 import 'package:file_picker/file_picker.dart';
@@ -40,6 +41,7 @@ import 'package:watermeter/page/setting/about_page/about_page.dart';
 import 'package:watermeter/page/setting/dialogs/experiment_password_dialog.dart';
 import 'package:watermeter/repository/pick_file.dart';
 import 'package:watermeter/repository/preference.dart' as preference;
+import 'package:watermeter/repository/system_calendar_sync_service.dart';
 import 'package:watermeter/page/setting/dialogs/electricity_password_dialog.dart';
 import 'package:watermeter/page/setting/dialogs/sport_password_dialog.dart';
 import 'package:watermeter/page/setting/dialogs/change_swift_dialog.dart';
@@ -84,6 +86,22 @@ class _SettingWindowState extends State<SettingWindow> {
     }
   }
 
+  bool get _isSemesterAwareControllerLoading =>
+      ClassTableController.i.schoolClassTableStateSignal.value.isLoading ||
+      ExamController.i.examInfoStateSignal.value.isLoading ||
+      PhysicsExperimentController.i.physicsExperimentStateSignal.value
+          .isLoading ||
+      OtherExperimentController.i.otherExperimentStateSignal.value.isLoading;
+
+  Future<void> _waitForSemesterAwareReloads() async {
+    await Future<void>.delayed(const Duration(milliseconds: 100));
+    final stopwatch = Stopwatch()..start();
+    while (_isSemesterAwareControllerLoading &&
+        stopwatch.elapsed < const Duration(seconds: 30)) {
+      await Future<void>.delayed(const Duration(milliseconds: 100));
+    }
+  }
+
   @override
   Widget build(BuildContext context) {
     List<String> demoBlueModeName = [
@@ -254,7 +272,8 @@ class _SettingWindowState extends State<SettingWindow> {
                   subtitle: Text(
                     FlutterI18n.translate(
                       context,
-                      "setting.change_color_dialog.${ColorSeed.values[preference.getInt(preference.Preference.color)].label}",
+                      "setting.change_color_dialog."
+                      "${ColorSeed.values[preference.getInt(preference.Preference.color)].label}",
                     ),
                   ),
                   trailing: const Icon(Icons.navigate_next),
@@ -737,6 +756,7 @@ class _SettingWindowState extends State<SettingWindow> {
                               OtherExperimentController.i
                                   .reloadOtherExperiment(),
                             ]);
+                            await maybeAutoSyncSystemCalendar();
                             if (mounted) {
                               setState(() {});
                             }
@@ -801,9 +821,17 @@ class _SettingWindowState extends State<SettingWindow> {
                       barrierDismissible: false,
                       context: context,
                       builder: (context) => SemesterSwitchDialog(),
-                    ).then((value) {
+                    ).then((value) async {
                       if (value == true) {
                         setState(() {});
+                        if (context.mounted) {
+                          showToast(context: context, msg: "Updating data");
+                        }
+                        await _waitForSemesterAwareReloads();
+                        await maybeAutoSyncSystemCalendar();
+                        if (mounted) {
+                          setState(() {});
+                        }
                       }
                     });
                   },
diff --git a/lib/repository/preference.dart b/lib/repository/preference.dart
index 59cdf4dd..4a6a951c 100644
--- a/lib/repository/preference.dart
+++ b/lib/repository/preference.dart
@@ -89,7 +89,16 @@ enum Preference {
   ), // 上次通知使用的语言
   dormWaterToken(key: "dorm_water_token", type: "String"), // 宿舍水机登录 token
   dormWaterUid(key: "dorm_water_uid", type: "String"), // 宿舍水机用户 uid
-  dormWaterEid(key: "dorm_water_eid", type: "String"); // 宿舍水机用户 eid
+  dormWaterEid(key: "dorm_water_eid", type: "String"), // 宿舍水机用户 eid
+  systemCalendarId(key: "system_calendar_id", type: "String"), // 导出的系统日历 id
+  systemCalendarSemesterCode(
+    key: "system_calendar_semester_code",
+    type: "String",
+  ), // 导出的系统日历绑定学期
+  systemCalendarSnapshot(
+    key: "system_calendar_snapshot",
+    type: "String",
+  ); // 上次同步到系统日历的数据快照
 
   const Preference({required this.key, this.type = "String"});
 
diff --git a/lib/repository/system_calendar_sync_service.dart b/lib/repository/system_calendar_sync_service.dart
new file mode 100644
index 00000000..38252c8e
--- /dev/null
+++ b/lib/repository/system_calendar_sync_service.dart
@@ -0,0 +1,553 @@
+// Copyright 2023-2025 BenderBlog Rodriguez and contributors
+// Copyright 2025 Traintime PDA authors.
+// SPDX-License-Identifier: MPL-2.0 OR Apache-2.0
+
+import 'dart:convert';
+
+import 'package:device_calendar/device_calendar.dart';
+import 'package:intl/intl.dart';
+import 'package:timezone/data/latest.dart' as tz;
+import 'package:watermeter/controller/classtable_controller.dart';
+import 'package:watermeter/controller/exam_controller.dart';
+import 'package:watermeter/controller/other_experiment_controller.dart';
+import 'package:watermeter/controller/physics_experiment_controller.dart';
+import 'package:watermeter/model/time_list.dart';
+import 'package:watermeter/model/xidian_ids/classtable.dart';
+import 'package:watermeter/model/xidian_ids/exam.dart';
+import 'package:watermeter/model/xidian_ids/experiment.dart';
+import 'package:watermeter/repository/logger.dart';
+import 'package:watermeter/repository/preference.dart' as preference;
+
+const String exportedClassTableCalendarPrefix = 'PDA Class Arrangement';
+
+/// Build a stable calendar name for exported classtable events.
+String buildExportedClassTableCalendarName(String semesterCode) =>
+    semesterCode.isEmpty
+        ? exportedClassTableCalendarPrefix
+        : '$exportedClassTableCalendarPrefix $semesterCode';
+
+/// Build a stable snapshot used to decide whether auto sync is needed.
+String buildSystemCalendarSnapshot({
+  required ClassTableData classTableData,
+  required List<Subject> subjects,
+  required List<ExperimentData> experiments,
+}) => jsonEncode({
+  'classTableData': classTableData.toJson(),
+  'subjects': subjects.map((item) => item.toJson()).toList(),
+  'experiments': experiments.map((item) => item.toJson()).toList(),
+});
+
+List<Event> buildCalendarEvents({
+  required ClassTableData classTableData,
+  required List<Subject> subjects,
+  required List<ExperimentData> experiments,
+}) {
+  List<Event> events = [];
+
+  tz.initializeTimeZones();
+  Location currentLocation = getLocation('Asia/Shanghai');
+
+  if (classTableData.termStartDay.isNotEmpty) {
+    DateTime startDay = DateTime.parse(classTableData.termStartDay);
+
+    for (var i in classTableData.timeArrangement) {
+      int j = i.weekList.indexWhere((element) => element);
+
+      if (j == -1) continue;
+
+      List<(int, int)> ranges = [];
+      int start = -1;
+
+      for (j; j < i.weekList.length; j++) {
+        if (i.weekList[j] && start == -1) {
+          start = j;
+        } else if (!i.weekList[j] && start != -1) {
+          ranges.add((start, j - 1));
+          start = -1;
+        }
+      }
+
+      if (start != -1) {
+        ranges.add((start, j - 1));
+      }
+
+      String title =
+          '${classTableData.getClassDetail(i).name}@${i.classroom ?? "待定"}';
+      String description =
+          '课程名称：${classTableData.getClassDetail(i).name} - 老师：${i.teacher ?? "未知"}';
+      String? location = i.classroom ?? '待定';
+
+      List<String> startTime = timeList[(i.start - 1) * 2].split(':');
+      List<String> stopTime = timeList[(i.stop - 1) * 2 + 1].split(':');
+
+      DayOfWeek getDayOfWeek(int day) {
+        switch (day) {
+          case 1:
+            return DayOfWeek.Monday;
+          case 2:
+            return DayOfWeek.Tuesday;
+          case 3:
+            return DayOfWeek.Wednesday;
+          case 4:
+            return DayOfWeek.Thursday;
+          case 5:
+            return DayOfWeek.Friday;
+          case 6:
+            return DayOfWeek.Saturday;
+          case 7:
+            return DayOfWeek.Sunday;
+          default:
+            return DayOfWeek.Sunday;
+        }
+      }
+
+      for (var range in ranges) {
+        DateTime firstDay = startDay.add(
+          Duration(days: range.$1 * 7 + i.day - 1),
+        );
+
+        DateTime startTimeToUse = firstDay.add(
+          Duration(
+            hours: int.parse(startTime[0]),
+            minutes: int.parse(startTime[1]),
+          ),
+        );
+        DateTime stopTimeToUse = firstDay.add(
+          Duration(
+            hours: int.parse(stopTime[0]),
+            minutes: int.parse(stopTime[1]),
+          ),
+        );
+
+        RecurrenceRule rrule = RecurrenceRule(
+          RecurrenceFrequency.Weekly,
+          daysOfWeek: [getDayOfWeek(i.day)],
+          endDate: firstDay.add(Duration(days: (range.$2 - range.$1) * 7 + 1)),
+        );
+
+        events.add(
+          Event(
+            null,
+            title: title,
+            description: description,
+            recurrenceRule: rrule,
+            start: TZDateTime.from(startTimeToUse, currentLocation),
+            end: TZDateTime.from(stopTimeToUse, currentLocation),
+            location: location,
+          ),
+        );
+      }
+    }
+  }
+
+  for (var i in subjects) {
+    String title =
+        '${i.subject} ${i.typeStr}@${i.place} ${i.seat != null ? "-${i.seat}" : ""}';
+    String description = '考试信息：${i.subject} - ${i.typeStr}';
+    String location = '${i.place} ${i.seat != null ? "-${i.seat}" : ""}';
+
+    events.add(
+      Event(
+        null,
+        title: title,
+        description: description,
+        start: TZDateTime.from(i.startTime!, currentLocation),
+        end: TZDateTime.from(i.stopTime!, currentLocation),
+        location: location,
+      ),
+    );
+  }
+
+  for (var experiment in experiments) {
+    for (var j in experiment.timeRanges) {
+      events.add(
+        Event(
+          null,
+          title: '${experiment.name}@${experiment.classroom}',
+          description: '实验名称：${experiment.name} - 老师：${experiment.teacher}',
+          start: TZDateTime.from(j.$1, currentLocation),
+          end: TZDateTime.from(j.$2, currentLocation),
+          location: experiment.classroom,
+        ),
+      );
+    }
+  }
+
+  return events;
+}
+
+String buildICalendarString(List<Event> events) {
+  String toReturn = '''BEGIN:VCALENDAR
+CALSCALE:GREGORIAN
+BEGIN:VTIMEZONE
+TZID:Asia/Shanghai
+X-LIC-LOCATION:Asia/Shanghai
+BEGIN:STANDARD
+TZOFFSETFROM:+0800
+TZOFFSETTO:+0800
+TZNAME:CST
+DTSTART:19700101T000000
+END:STANDARD
+END:VTIMEZONE
+''';
+
+  for (var i in events) {
+    String vevent = 'BEGIN:VEVENT\n';
+
+    vevent +=
+        'DTSTAMP:${DateFormat('yyyyMMddTHHmmssZ').format(DateTime.now())}\n';
+    vevent += 'SUMMARY:${i.title ?? "待定"}\n';
+    vevent += 'DESCRIPTION:${i.description ?? "待定"}\n';
+
+    vevent +=
+        'DTSTART;TZID=Asia/Shanghai:'
+        '${DateFormat('yyyyMMddTHHmmss').format(
+          DateTime.fromMicrosecondsSinceEpoch(i.start!.microsecondsSinceEpoch),
+        )}\n';
+    vevent +=
+        'DTEND;TZID=Asia/Shanghai:'
+        '${DateFormat('yyyyMMddTHHmmss').format(
+          DateTime.fromMicrosecondsSinceEpoch(i.end!.microsecondsSinceEpoch),
+        )}\n';
+    if (i.location != null) {
+      vevent += 'LOCATION:${i.location}\n';
+    }
+
+    if (i.recurrenceRule != null) {
+      String getWeekStr(DayOfWeek day) {
+        switch (day) {
+          case DayOfWeek.Monday:
+            return 'MO';
+          case DayOfWeek.Tuesday:
+            return 'TU';
+          case DayOfWeek.Wednesday:
+            return 'WE';
+          case DayOfWeek.Thursday:
+            return 'TH';
+          case DayOfWeek.Friday:
+            return 'FR';
+          case DayOfWeek.Saturday:
+            return 'SA';
+          case DayOfWeek.Sunday:
+            return 'SU';
+        }
+      }
+
+      vevent +=
+          'RRULE:FREQ=WEEKLY;INTERVAL=1;BYDAY=${getWeekStr(i.recurrenceRule!.daysOfWeek!.first)};'
+          'UNTIL=${DateFormat('yyyyMMdd').format(i.recurrenceRule!.endDate!)}\n';
+    }
+    toReturn += '${vevent}END:VEVENT\n';
+  }
+
+  return '${toReturn}END:VCALENDAR';
+}
+
+class SystemCalendarSyncService {
+  final DeviceCalendarPlugin deviceCalendarPlugin = DeviceCalendarPlugin();
+
+  ClassTableController get classTableController => ClassTableController.i;
+  ExamController get examController => ExamController.i;
+  PhysicsExperimentController get physicsExperimentController =>
+      PhysicsExperimentController.i;
+  OtherExperimentController get otherExperimentController =>
+      OtherExperimentController.i;
+
+  String get currentSemesterCode =>
+      classTableController.classTableComputedSignal.value.semesterCode;
+
+  String get savedCalendarId =>
+      preference.getString(preference.Preference.systemCalendarId);
+
+  String get savedCalendarSemesterCode =>
+      preference.getString(preference.Preference.systemCalendarSemesterCode);
+
+  String get calendarName => buildExportedClassTableCalendarName(
+    currentSemesterCode,
+  );
+
+  List<ExperimentData> get experiments => [
+    ...physicsExperimentController.physicsExperiments.value,
+    ...otherExperimentController.otherExperiments.value,
+  ];
+
+  List<Event> get events => buildCalendarEvents(
+    classTableData: classTableController.classTableComputedSignal.value,
+    subjects: examController.subjects.value,
+    experiments: experiments,
+  );
+
+  String get snapshot => buildSystemCalendarSnapshot(
+    classTableData: classTableController.classTableComputedSignal.value,
+    subjects: examController.subjects.value,
+    experiments: experiments,
+  );
+
+  bool get hasCalendarBinding => savedCalendarId.isNotEmpty;
+
+  bool get isBoundCalendarForCurrentSemester =>
+      savedCalendarSemesterCode.isNotEmpty &&
+      savedCalendarSemesterCode == currentSemesterCode;
+
+  bool get didSemesterChangeSinceLastBinding =>
+      savedCalendarSemesterCode.isNotEmpty &&
+      currentSemesterCode.isNotEmpty &&
+      savedCalendarSemesterCode != currentSemesterCode;
+
+  bool get canAutoSync =>
+      hasCalendarBinding &&
+      (didSemesterChangeSinceLastBinding ||
+          preference.getString(preference.Preference.systemCalendarSnapshot) !=
+              snapshot) &&
+      classTableController.hasValidClassInfo.value;
+
+  Future<bool> _ensureCalendarPermission({
+    required bool requestPermissionsIfNeeded,
+    Future<void> Function()? showDialog,
+  }) async {
+    Result<bool> hasPermitted = await deviceCalendarPlugin.hasPermissions();
+    if (hasPermitted.data == true) {
+      return true;
+    }
+
+    if (!requestPermissionsIfNeeded) {
+      return false;
+    }
+
+    if (showDialog != null) {
+      await showDialog();
+    }
+    hasPermitted = await deviceCalendarPlugin.requestPermissions();
+    return hasPermitted.data == true;
+  }
+
+  Future<void> _saveCalendarBinding(String calendarId) async {
+    await preference.setString(
+      preference.Preference.systemCalendarId,
+      calendarId,
+    );
+    await preference.setString(
+      preference.Preference.systemCalendarSemesterCode,
+      currentSemesterCode,
+    );
+  }
+
+  Future<void> _saveCalendarSyncState(String calendarId) async {
+    await _saveCalendarBinding(calendarId);
+    await preference.setString(
+      preference.Preference.systemCalendarSnapshot,
+      snapshot,
+    );
+  }
+
+  Future<void> _clearCalendarBinding() async {
+    await preference.remove(preference.Preference.systemCalendarId);
+    await preference.remove(preference.Preference.systemCalendarSemesterCode);
+  }
+
+  Future<List<Calendar>?> _retrieveWritableCalendars() async {
+    final calendarResult = await deviceCalendarPlugin.retrieveCalendars();
+    if (!calendarResult.isSuccess || calendarResult.data == null) {
+      log.info(
+        '[SystemCalendarSyncService][_retrieveWritableCalendars] '
+        'Retrieve calendars failed.',
+      );
+      return null;
+    }
+
+    return calendarResult.data!
+        .where((calendar) => calendar.isReadOnly != true)
+        .toList();
+  }
+
+  Future<Calendar?> _findBoundCalendar(List<Calendar> calendars) async {
+    if (!hasCalendarBinding) {
+      return null;
+    }
+
+    for (var calendar in calendars) {
+      if (calendar.id != savedCalendarId) {
+        continue;
+      }
+
+      if (isBoundCalendarForCurrentSemester) {
+        return calendar;
+      }
+
+      // Migrate older bindings that do not persist the semester code yet.
+      if (savedCalendarSemesterCode.isEmpty &&
+          calendar.name == calendarName &&
+          calendar.id != null &&
+          calendar.id!.isNotEmpty) {
+        await _saveCalendarBinding(calendar.id!);
+        return calendar;
+      }
+
+      log.info(
+        '[SystemCalendarSyncService][_findBoundCalendar] '
+        'Skip maintaining bound calendar from semester '
+        '$savedCalendarSemesterCode while current semester is '
+        '$currentSemesterCode.',
+      );
+      return null;
+    }
+
+    await _clearCalendarBinding();
+    return null;
+  }
+
+  /// Find the exported calendar by saved id or the current fixed name.
+  Future<Calendar?> _findExportedCalendar() async {
+    final calendars = await _retrieveWritableCalendars();
+    if (calendars == null) {
+      return null;
+    }
+
+    final hadCalendarBinding = hasCalendarBinding;
+    final boundCalendar = await _findBoundCalendar(calendars);
+    if (boundCalendar != null) {
+      return boundCalendar;
+    }
+    // Treat a missing bound calendar as an explicit user opt-out, so auto sync
+    // does not resurrect the export by falling back to name lookup.
+    if (hadCalendarBinding && !hasCalendarBinding) {
+      return null;
+    }
+
+    for (var calendar in calendars) {
+      if (calendar.name == calendarName &&
+          calendar.id != null &&
+          calendar.id!.isNotEmpty) {
+        await _saveCalendarBinding(calendar.id!);
+        return calendar;
+      }
+    }
+
+    return null;
+  }
+
+  Future<String?> _createExportedCalendar() async {
+    Result<String> calendarIdData = await deviceCalendarPlugin.createCalendar(
+      calendarName,
+    );
+    if (!calendarIdData.isSuccess ||
+        calendarIdData.data == null ||
+        calendarIdData.data!.isEmpty) {
+      log.info(
+        '[SystemCalendarSyncService][_createExportedCalendar] '
+        'Create calendar failed.',
+      );
+      return null;
+    }
+
+    return calendarIdData.data!;
+  }
+
+  Future<String?> _prepareCalendar({required bool onlyIfCalendarExists}) async {
+    Calendar? exportedCalendar = await _findExportedCalendar();
+    bool shouldCreateCurrentSemesterCalendar =
+        !onlyIfCalendarExists || didSemesterChangeSinceLastBinding;
+
+    if (exportedCalendar == null) {
+      if (!shouldCreateCurrentSemesterCalendar) {
+        return null;
+      }
+      return await _createExportedCalendar();
+    }
+
+    if (exportedCalendar.id == null || exportedCalendar.id!.isEmpty) {
+      log.info(
+        '[SystemCalendarSyncService][_prepareCalendar] '
+        'Exported calendar has empty id.',
+      );
+      return null;
+    }
+
+    Result<bool> deleteResult = await deviceCalendarPlugin.deleteCalendar(
+      exportedCalendar.id!,
+    );
+    if (deleteResult.data != true) {
+      log.info(
+        '[SystemCalendarSyncService][_prepareCalendar] '
+        'Delete old exported calendar failed.',
+      );
+      return null;
+    }
+
+    await _clearCalendarBinding();
+    return await _createExportedCalendar();
+  }
+
+  Future<bool> _writeEventsToCalendar(String calendarId) async {
+    for (var i in events) {
+      final toPush = i..calendarId = calendarId;
+      Result<String>? addEventResult = await deviceCalendarPlugin
+          .createOrUpdateEvent(toPush);
+      if (addEventResult == null ||
+          addEventResult.data == null ||
+          addEventResult.data!.isEmpty) {
+        log.info(
+          '[SystemCalendarSyncService][_writeEventsToCalendar] '
+          'Write event failed.',
+        );
+        return false;
+      }
+    }
+
+    return true;
+  }
+
+  Future<bool> syncSystemCalendar({
+    required bool requestPermissionsIfNeeded,
+    required bool onlyIfCalendarExists,
+    Future<void> Function()? showDialog,
+  }) async {
+    try {
+      bool hasPermitted = await _ensureCalendarPermission(
+        requestPermissionsIfNeeded: requestPermissionsIfNeeded,
+        showDialog: showDialog,
+      );
+      if (!hasPermitted) {
+        log.info(
+          '[SystemCalendarSyncService][syncSystemCalendar] '
+          'Calendar permission denied.',
+        );
+        return false;
+      }
+
+      String? calendarId = await _prepareCalendar(
+        onlyIfCalendarExists: onlyIfCalendarExists,
+      );
+      if (calendarId == null || calendarId.isEmpty) {
+        return false;
+      }
+
+      bool didWrite = await _writeEventsToCalendar(calendarId);
+      if (didWrite) {
+        await _saveCalendarSyncState(calendarId);
+      }
+      return didWrite;
+    } catch (e, s) {
+      log.handle(e, s);
+      return false;
+    }
+  }
+}
+
+/// Auto sync only when the exported payload changed and the bound calendar
+/// still exists.
+Future<void> maybeAutoSyncSystemCalendar() async {
+  try {
+    final service = SystemCalendarSyncService();
+    if (!service.canAutoSync) {
+      return;
+    }
+
+    await service.syncSystemCalendar(
+      requestPermissionsIfNeeded: false,
+      onlyIfCalendarExists: true,
+    );
+  } catch (e, s) {
+    log.handle(e, s);
+  }
+}