diff --git a/sync.js b/sync.js
new file mode 100644
index 0000000..9341be3
--- /dev/null
+++ b/sync.js
@@ -0,0 +1,107 @@
+/**
+ * DataSync — hält LocalStorage (synchrone Working-Copy) und Server-DB
+ * (dauerhafte Wahrheit + Backup + Historie) synchron.
+ * ponytail: last-write-wins auf ganzen Dokumenten. Single-User → genügt.
+ */
+const DataSync = {
+  KEY_EMPLOYEES: 'dienstplan_employees',
+  KEY_DUTIES: 'dienstplan_duties',
+  KEY_VACATION: 'dienstplan_vacation',
+  KEY_PENDING: 'dienstplan_sync_pending',
+  _applying: false,
+  _timer: null,
+  online: false,
+
+  _local() {
+    const parse = (k, fb) => {
+      try { const v = JSON.parse(localStorage.getItem(k)); return v ?? fb; }
+      catch { return fb; }
+    };
+    return {
+      employees: parse(this.KEY_EMPLOYEES, []),
+      duties: parse(this.KEY_DUTIES, {}),
+      vacation: parse(this.KEY_VACATION, {}),
+    };
+  },
+
+  _isEmpty(s) {
+    return (!s.employees || s.employees.length === 0)
+      && (!s.duties || Object.keys(s.duties).length === 0)
+      && (!s.vacation || Object.keys(s.vacation).length === 0);
+  },
+
+  // Reine Entscheidung — in Node unit-getestet.
+  decideSync(local, server, pending) {
+    if (server === null) return 'offline';
+    if (pending) return 'push-local';
+    if (this._isEmpty(server) && !this._isEmpty(local)) return 'push-local';
+    return 'adopt-server';
+  },
+
+  _applyServer(server) {
+    this._applying = true;
+    try {
+      localStorage.setItem(this.KEY_EMPLOYEES, JSON.stringify(server.employees ?? []));
+      localStorage.setItem(this.KEY_DUTIES, JSON.stringify(server.duties ?? {}));
+      localStorage.setItem(this.KEY_VACATION, JSON.stringify(server.vacation ?? {}));
+    } finally {
+      this._applying = false;
+    }
+  },
+
+  async boot() {
+    let server = null;
+    try {
+      const res = await fetch('/api/state', { cache: 'no-store' });
+      if (res.ok) server = await res.json();
+    } catch { /* offline */ }
+
+    const action = this.decideSync(this._local(), server, localStorage.getItem(this.KEY_PENDING) === '1');
+    if (action === 'offline') { this.online = false; this._renderStatus(); return; }
+    this.online = true;
+    if (action === 'adopt-server') this._applyServer(server);
+    else await this._flush(); // push-local: Erstmigration oder pending Offline-Aenderungen
+    this._renderStatus();
+  },
+
+  push() {
+    if (this._applying) return;
+    localStorage.setItem(this.KEY_PENDING, '1');
+    clearTimeout(this._timer);
+    this._timer = setTimeout(() => this._flush(), 500);
+  },
+
+  async _flush() {
+    try {
+      const res = await fetch('/api/state', {
+        method: 'PUT',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify(this._local()),
+      });
+      if (!res.ok) throw new Error('HTTP ' + res.status);
+      localStorage.removeItem(this.KEY_PENDING); // erst nach Erfolg
+      this.online = true;
+    } catch (e) {
+      console.error('Sync fehlgeschlagen, Daten bleiben lokal:', e);
+      this.online = false; // pending bleibt gesetzt → naechster boot() pusht
+    }
+    this._renderStatus();
+  },
+
+  _renderStatus() {
+    if (typeof document === 'undefined' || !document.body) return;
+    let el = document.getElementById('sync-status');
+    if (!el) {
+      el = document.createElement('div');
+      el.id = 'sync-status';
+      el.style.cssText = 'position:fixed;bottom:8px;right:8px;font-size:12px;'
+        + 'padding:4px 8px;border-radius:6px;z-index:9999;opacity:.85;color:#fff';
+      document.body.appendChild(el);
+    }
+    el.textContent = this.online ? '● Synchronisiert' : '● Offline (nur lokal)';
+    el.style.background = this.online ? '#1e7e34' : '#856404';
+  },
+};
+
+if (typeof module !== 'undefined' && module.exports) module.exports = DataSync;
+if (typeof window !== 'undefined') window.DataSync = DataSync;
diff --git a/sync.test.js b/sync.test.js
new file mode 100644
index 0000000..1d5d25b
--- /dev/null
+++ b/sync.test.js
@@ -0,0 +1,26 @@
+const { test } = require('node:test');
+const assert = require('node:assert');
+const DataSync = require('./sync');
+
+const empty = { employees: [], duties: {}, vacation: {} };
+const full = { employees: ['Max'], duties: { Max: {} }, vacation: {} };
+
+test('Server unerreichbar → offline', () => {
+  assert.strictEqual(DataSync.decideSync(full, null, false), 'offline');
+});
+
+test('pending gesetzt → push-local (Offline-Aenderungen gewinnen)', () => {
+  assert.strictEqual(DataSync.decideSync(full, full, true), 'push-local');
+});
+
+test('Server leer, lokal voll → push-local (Erstmigration)', () => {
+  assert.strictEqual(DataSync.decideSync(full, empty, false), 'push-local');
+});
+
+test('Server hat Daten → adopt-server', () => {
+  assert.strictEqual(DataSync.decideSync(empty, full, false), 'adopt-server');
+});
+
+test('beide leer → adopt-server (nichts zu tun)', () => {
+  assert.strictEqual(DataSync.decideSync(empty, empty, false), 'adopt-server');
+});