diff --git a/frontend/src/features/reminder/domain/geofence.ts b/frontend/src/features/reminder/domain/geofence.ts
new file mode 100644
index 0000000..ad75edb
--- /dev/null
+++ b/frontend/src/features/reminder/domain/geofence.ts
@@ -0,0 +1,67 @@
+import type { GeoPoint, LocalReminderSchedule } from './reminder';
+
+const EARTH_RADIUS_METERS = 6_371_000;
+
+export type GeofenceWatchMode = 'arrive' | 'return';
+export type GeofenceTransition = 'no_change' | 'armed' | 'triggered';
+
+/** Haversine 球面距离（米）。 */
+export function distanceMeters(from: GeoPoint | null | undefined, to: GeoPoint): number {
+  if (from == null || from.latitude == null || from.longitude == null) {
+    return Number.POSITIVE_INFINITY;
+  }
+  const phi1 = toRadians(from.latitude);
+  const phi2 = toRadians(to.latitude);
+  const deltaPhi = toRadians(to.latitude - from.latitude);
+  const deltaLambda = toRadians(to.longitude - from.longitude);
+  const a =
+    Math.sin(deltaPhi / 2) ** 2 + Math.cos(phi1) * Math.cos(phi2) * Math.sin(deltaLambda / 2) ** 2;
+  return 2 * EARTH_RADIUS_METERS * Math.asin(Math.sqrt(a));
+}
+
+/**
+ * 本地围栏边沿状态机：
+ * - geofence_armed=false：在圈内未离开；离开后变为 armed
+ * - geofence_armed=true：再次进入才 triggered
+ * - mode 决定中心点语义（arrive=日程坐标，return=记录点）
+ */
+export function evaluateGeofence(
+  schedule: LocalReminderSchedule,
+  sample: GeoPoint,
+  mode: GeofenceWatchMode = resolveWatchMode(schedule),
+): GeofenceTransition {
+  const center = resolveGeofenceCenter(schedule, mode);
+  if (center == null) return 'no_change';
+
+  const inside = distanceMeters(center, sample) <= schedule.geofence_radius_meters;
+  const armed = schedule.runtime.geofence_armed;
+
+  if (armed) {
+    return inside ? 'triggered' : 'no_change';
+  }
+  return inside ? 'no_change' : 'armed';
+}
+
+export function resolveWatchMode(schedule: LocalReminderSchedule): GeofenceWatchMode {
+  return schedule.reminder?.reminder_type === 'return_to_recorded_location' ? 'return' : 'arrive';
+}
+
+export function resolveGeofenceCenter(
+  schedule: LocalReminderSchedule,
+  mode: GeofenceWatchMode = resolveWatchMode(schedule),
+): GeoPoint | null {
+  if (mode === 'return') {
+    if (schedule.runtime.recorded_location != null) {
+      return schedule.runtime.recorded_location;
+    }
+    return null;
+  }
+  if (schedule.latitude == null || schedule.longitude == null) {
+    return null;
+  }
+  return { latitude: schedule.latitude, longitude: schedule.longitude };
+}
+
+function toRadians(degrees: number): number {
+  return (degrees * Math.PI) / 180;
+}
diff --git a/frontend/src/features/reminder/domain/index.ts b/frontend/src/features/reminder/domain/index.ts
index 0ce175c..86f8740 100644
--- a/frontend/src/features/reminder/domain/index.ts
+++ b/frontend/src/features/reminder/domain/index.ts
@@ -20,3 +20,18 @@ export type {
   ReminderType,
 } from './reminder';
 export { DEFAULT_SNOOZE_MINUTES } from './reminder';
+export type { GeofenceTransition, GeofenceWatchMode } from './geofence';
+export {
+  distanceMeters,
+  evaluateGeofence,
+  resolveGeofenceCenter,
+  resolveWatchMode,
+} from './geofence';
+export {
+  isSnoozeActive,
+  isSnoozeExpired,
+  isTimeWindowReached,
+  resolveEffectiveTriggerAt,
+  resolveSnoozeUntil,
+  resolveTimeTriggerAt,
+} from './timeWindow';
diff --git a/frontend/src/features/reminder/domain/timeWindow.ts b/frontend/src/features/reminder/domain/timeWindow.ts
new file mode 100644
index 0000000..9a44c52
--- /dev/null
+++ b/frontend/src/features/reminder/domain/timeWindow.ts
@@ -0,0 +1,73 @@
+import { DEFAULT_SNOOZE_MINUTES, type LocalReminderSchedule } from './reminder';
+
+/** 计算时间型提醒的应触发时刻（ISO）。无有效时间则返回 null。 */
+export function resolveTimeTriggerAt(schedule: LocalReminderSchedule): string | null {
+  const reminder = schedule.reminder;
+  if (reminder == null) return null;
+
+  if (reminder.reminder_trigger_at != null) {
+    return reminder.reminder_trigger_at;
+  }
+
+  if (schedule.start_time == null) return null;
+
+  if (reminder.reminder_type === 'before_start') {
+    const offsetMinutes = reminder.reminder_offset_minutes ?? 0;
+    const startMs = Date.parse(schedule.start_time);
+    if (Number.isNaN(startMs)) return null;
+    return new Date(startMs - offsetMinutes * 60_000).toISOString();
+  }
+
+  if (reminder.reminder_type === 'at_time') {
+    return schedule.start_time;
+  }
+
+  return null;
+}
+
+/** 当前有效触发时刻：snooze 未结束时以 snoozed_until 为准。 */
+export function resolveEffectiveTriggerAt(schedule: LocalReminderSchedule): string | null {
+  if (
+    schedule.runtime.reminder_disposition_state === 'snoozed' &&
+    schedule.runtime.snoozed_until != null
+  ) {
+    return schedule.runtime.snoozed_until;
+  }
+  return resolveTimeTriggerAt(schedule);
+}
+
+export function resolveSnoozeUntil(
+  nowIso: string,
+  snoozeUntil: string | null | undefined,
+  snoozeMinutes: number | null | undefined,
+): string {
+  if (snoozeUntil != null) return snoozeUntil;
+  const minutes = snoozeMinutes ?? DEFAULT_SNOOZE_MINUTES;
+  const nowMs = Date.parse(nowIso);
+  const base = Number.isNaN(nowMs) ? Date.now() : nowMs;
+  return new Date(base + minutes * 60_000).toISOString();
+}
+
+export function isSnoozeActive(schedule: LocalReminderSchedule, nowIso: string): boolean {
+  if (schedule.runtime.reminder_disposition_state !== 'snoozed') return false;
+  const until = schedule.runtime.snoozed_until;
+  if (until == null) return false;
+  return Date.parse(nowIso) < Date.parse(until);
+}
+
+export function isTimeWindowReached(schedule: LocalReminderSchedule, nowIso: string): boolean {
+  if (schedule.status !== 'active') return false;
+  if (schedule.schedule_type !== 'time') return false;
+  if (isSnoozeActive(schedule, nowIso)) return false;
+  if (schedule.runtime.reminder_disposition_state === 'snoozed') return false;
+  const triggerAt = resolveEffectiveTriggerAt(schedule);
+  if (triggerAt == null) return false;
+  return Date.parse(nowIso) >= Date.parse(triggerAt);
+}
+
+export function isSnoozeExpired(schedule: LocalReminderSchedule, nowIso: string): boolean {
+  const until = schedule.runtime.snoozed_until;
+  if (until == null) return false;
+  if (schedule.runtime.reminder_disposition_state !== 'snoozed') return false;
+  return Date.parse(nowIso) >= Date.parse(until);
+}