diff --git a/src/lib/storage.js b/src/lib/storage.js
new file mode 100644
index 0000000..29ced0e
--- /dev/null
+++ b/src/lib/storage.js
@@ -0,0 +1,144 @@
+import { isKey, toKey } from './dates.js'
+
+const TASK_PREFIX = 'negotium-tasks-'
+const THEME_KEY = 'negotium-theme'
+
+/** Backend used in tests, and as the production fallback when localStorage is
+ *  unavailable (Safari private mode, disabled storage, exhausted quota). */
+export function createMemoryStore() {
+  const map = new Map()
+  return {
+    getItem: (key) => (map.has(key) ? map.get(key) : null),
+    setItem: (key, value) => {
+      map.set(key, String(value))
+    },
+    removeItem: (key) => {
+      map.delete(key)
+    },
+    keys: () => [...map.keys()],
+  }
+}
+
+function wrapWebStorage(webStorage) {
+  return {
+    getItem: (key) => webStorage.getItem(key),
+    setItem: (key, value) => webStorage.setItem(key, value),
+    removeItem: (key) => webStorage.removeItem(key),
+    keys: () => Object.keys(webStorage),
+  }
+}
+
+function resolveBackend() {
+  try {
+    const probe = '__negotium_probe__'
+    window.localStorage.setItem(probe, '1')
+    window.localStorage.removeItem(probe)
+    return wrapWebStorage(window.localStorage)
+  } catch {
+    return createMemoryStore()
+  }
+}
+
+export function createStorage(backend) {
+  const store = backend ?? resolveBackend()
+
+  function read(key) {
+    try {
+      return store.getItem(key)
+    } catch {
+      return null
+    }
+  }
+
+  function write(key, value) {
+    try {
+      store.setItem(key, value)
+    } catch {
+      // Quota exhausted or storage revoked mid-session. In-memory state stays
+      // authoritative; losing a write beats losing the app.
+    }
+  }
+
+  function drop(key) {
+    try {
+      store.removeItem(key)
+    } catch {
+      // Nothing actionable.
+    }
+  }
+
+  function allKeys() {
+    try {
+      return store.keys()
+    } catch {
+      return []
+    }
+  }
+
+  function parseTasks(raw) {
+    if (!raw) return []
+    try {
+      const parsed = JSON.parse(raw)
+      return Array.isArray(parsed) ? parsed : []
+    } catch {
+      return []
+    }
+  }
+
+  function loadTasks(dateKey) {
+    return parseTasks(read(TASK_PREFIX + dateKey))
+  }
+
+  function saveTasks(dateKey, tasks) {
+    write(TASK_PREFIX + dateKey, JSON.stringify(tasks))
+  }
+
+  function taskSuffixes() {
+    return allKeys()
+      .filter((key) => key.startsWith(TASK_PREFIX))
+      .map((key) => key.slice(TASK_PREFIX.length))
+  }
+
+  function listTaskKeys() {
+    return taskSuffixes().filter(isKey)
+  }
+
+  function removeTasks(dateKey) {
+    drop(TASK_PREFIX + dateKey)
+  }
+
+  function loadTheme() {
+    return read(THEME_KEY)
+  }
+
+  function saveTheme(mode) {
+    write(THEME_KEY, mode)
+  }
+
+  /** One-time conversion of `negotium-tasks-Sat Aug 15 2026` keys written by
+   *  versions before the ISO format. Idempotent, and deliberately conservative:
+   *  a suffix that will not parse is left in place rather than discarded. */
+  function migrateLegacyKeys() {
+    for (const suffix of taskSuffixes().filter((s) => !isKey(s))) {
+      const parsed = new Date(suffix)
+      if (Number.isNaN(parsed.getTime())) continue
+
+      const isoKey = toKey(parsed)
+      const legacy = parseTasks(read(TASK_PREFIX + suffix))
+      const existing = loadTasks(isoKey)
+
+      saveTasks(isoKey, [...legacy, ...existing])
+      drop(TASK_PREFIX + suffix)
+    }
+  }
+
+  return {
+    loadTasks,
+    saveTasks,
+    listTaskKeys,
+    removeTasks,
+    loadTheme,
+    saveTheme,
+    migrateLegacyKeys,
+  }
+}
diff --git a/src/lib/storage.test.js b/src/lib/storage.test.js
new file mode 100644
index 0000000..959bd97
--- /dev/null
+++ b/src/lib/storage.test.js
@@ -0,0 +1,170 @@
+import { describe, it, expect } from 'vitest'
+import { createStorage, createMemoryStore } from './storage.js'
+
+const setup = (seed = {}) => {
+  const backend = createMemoryStore()
+  for (const [key, value] of Object.entries(seed)) backend.setItem(key, value)
+  return { backend, storage: createStorage(backend) }
+}
+
+describe('loadTasks', () => {
+  it('returns an empty array for a missing key', () => {
+    const { storage } = setup()
+    expect(storage.loadTasks('2026-08-15')).toEqual([])
+  })
+
+  it('returns stored tasks', () => {
+    const tasks = [{ id: 'a', text: 'buy milk', completed: false }]
+    const { storage } = setup({ 'negotium-tasks-2026-08-15': JSON.stringify(tasks) })
+    expect(storage.loadTasks('2026-08-15')).toEqual(tasks)
+  })
+
+  it('returns an empty array for corrupt JSON instead of throwing', () => {
+    const { storage } = setup({ 'negotium-tasks-2026-08-15': '{not json' })
+    expect(storage.loadTasks('2026-08-15')).toEqual([])
+  })
+
+  it('returns an empty array when the stored value is not an array', () => {
+    const { storage } = setup({ 'negotium-tasks-2026-08-15': '{"a":1}' })
+    expect(storage.loadTasks('2026-08-15')).toEqual([])
+  })
+
+  it('survives a throwing backend', () => {
+    const backend = createMemoryStore()
+    backend.getItem = () => {
+      throw new Error('storage revoked')
+    }
+    const storage = createStorage(backend)
+    expect(storage.loadTasks('2026-08-15')).toEqual([])
+  })
+})
+
+describe('saveTasks', () => {
+  it('round-trips through the backend', () => {
+    const { storage } = setup()
+    const tasks = [{ id: 'a', text: 'x', completed: true }]
+    storage.saveTasks('2026-08-15', tasks)
+    expect(storage.loadTasks('2026-08-15')).toEqual(tasks)
+  })
+
+  it('swallows a throwing backend rather than propagating', () => {
+    const backend = createMemoryStore()
+    backend.setItem = () => {
+      throw new Error('QuotaExceededError')
+    }
+    const storage = createStorage(backend)
+    expect(() => storage.saveTasks('2026-08-15', [])).not.toThrow()
+  })
+})
+
+describe('listTaskKeys', () => {
+  it('returns only date keys, with the prefix stripped', () => {
+    const { storage } = setup({
+      'negotium-tasks-2026-08-15': '[]',
+      'negotium-tasks-2026-08-16': '[]',
+      'negotium-theme': 'dark',
+      unrelated: 'x',
+    })
+    expect(storage.listTaskKeys().sort()).toEqual(['2026-08-15', '2026-08-16'])
+  })
+
+  it('excludes legacy-format keys', () => {
+    const { storage } = setup({
+      'negotium-tasks-2026-08-15': '[]',
+      'negotium-tasks-Sat Aug 15 2026': '[]',
+    })
+    expect(storage.listTaskKeys()).toEqual(['2026-08-15'])
+  })
+
+  it('returns an empty array when enumeration throws', () => {
+    const backend = createMemoryStore()
+    backend.keys = () => {
+      throw new Error('nope')
+    }
+    expect(createStorage(backend).listTaskKeys()).toEqual([])
+  })
+})
+
+describe('removeTasks', () => {
+  it('deletes the entry', () => {
+    const { storage } = setup({ 'negotium-tasks-2026-08-15': '[]' })
+    storage.removeTasks('2026-08-15')
+    expect(storage.listTaskKeys()).toEqual([])
+  })
+})
+
+describe('theme', () => {
+  it('returns null when unset', () => {
+    const { storage } = setup()
+    expect(storage.loadTheme()).toBe(null)
+  })
+
+  it('round-trips', () => {
+    const { storage } = setup()
+    storage.saveTheme('dark')
+    expect(storage.loadTheme()).toBe('dark')
+  })
+})
+
+describe('migrateLegacyKeys', () => {
+  it('rewrites a legacy key to ISO form', () => {
+    const tasks = [{ id: 'a', text: 'legacy', completed: false }]
+    const { storage } = setup({ 'negotium-tasks-Sat Aug 15 2026': JSON.stringify(tasks) })
+    storage.migrateLegacyKeys()
+    expect(storage.listTaskKeys()).toEqual(['2026-08-15'])
+    expect(storage.loadTasks('2026-08-15')).toEqual(tasks)
+  })
+
+  it('removes the legacy key once migrated', () => {
+    const { backend, storage } = setup({ 'negotium-tasks-Sat Aug 15 2026': '[]' })
+    storage.migrateLegacyKeys()
+    expect(backend.getItem('negotium-tasks-Sat Aug 15 2026')).toBe(null)
+  })
+
+  it('merges legacy before existing when the ISO key is occupied', () => {
+    const legacy = [{ id: 'l', text: 'legacy', completed: false }]
+    const current = [{ id: 'c', text: 'current', completed: false }]
+    const { storage } = setup({
+      'negotium-tasks-Sat Aug 15 2026': JSON.stringify(legacy),
+      'negotium-tasks-2026-08-15': JSON.stringify(current),
+    })
+    storage.migrateLegacyKeys()
+    expect(storage.loadTasks('2026-08-15').map((t) => t.id)).toEqual(['l', 'c'])
+  })
+
+  it('leaves an unparseable key untouched rather than destroying it', () => {
+    const { storage, backend } = setup({ 'negotium-tasks-not-a-date': '[{"id":"x"}]' })
+    storage.migrateLegacyKeys()
+    expect(backend.getItem('negotium-tasks-not-a-date')).toBe('[{"id":"x"}]')
+  })
+
+  it('is idempotent', () => {
+    const { storage } = setup({ 'negotium-tasks-Sat Aug 15 2026': '[{"id":"a"}]' })
+    storage.migrateLegacyKeys()
+    storage.migrateLegacyKeys()
+    expect(storage.loadTasks('2026-08-15')).toEqual([{ id: 'a' }])
+  })
+
+  it('migrates several legacy days independently', () => {
+    const { storage } = setup({
+      'negotium-tasks-Fri Aug 14 2026': '[{"id":"fri"}]',
+      'negotium-tasks-Sat Aug 15 2026': '[{"id":"sat"}]',
+    })
+    storage.migrateLegacyKeys()
+    expect(storage.listTaskKeys().sort()).toEqual(['2026-08-14', '2026-08-15'])
+    expect(storage.loadTasks('2026-08-14')).toEqual([{ id: 'fri' }])
+    expect(storage.loadTasks('2026-08-15')).toEqual([{ id: 'sat' }])
+  })
+
+  it('does nothing when there is nothing to migrate', () => {
+    const { storage } = setup({ 'negotium-tasks-2026-08-15': '[]' })
+    storage.migrateLegacyKeys()
+    expect(storage.listTaskKeys()).toEqual(['2026-08-15'])
+  })
+
+  it('leaves the theme key alone', () => {
+    const { backend, storage } = setup({ 'negotium-theme': 'dark' })
+    storage.migrateLegacyKeys()
+    expect(backend.getItem('negotium-theme')).toBe('dark')
+  })
+})