diff --git a/src/lib/date.js b/src/lib/date.js
new file mode 100644
index 0000000..3f47198
--- /dev/null
+++ b/src/lib/date.js
@@ -0,0 +1,59 @@
+/**
+ * Centralized date-string utilities.
+ *
+ * All date values in the app use the 'YYYY-MM-DD' (wall-clock) format.
+ * These helpers are the single source of truth for formatting and
+ * arithmetic on that format, replacing ad-hoc inline code that was
+ * duplicated across organize.js, recur.js, useCalendar.js, and ical-io.js.
+ */
+
+/** Pad a number to 2 digits with leading zero. */
+export function pad2(n) {
+  return String(n).padStart(2, '0')
+}
+
+/** Convert a JS Date to 'YYYY-MM-DD' (local wall-clock). */
+export function toISODate(date) {
+  return `${date.getFullYear()}-${pad2(date.getMonth() + 1)}-${pad2(date.getDate())}`
+}
+
+/** Parse 'YYYY-MM-DD' into { year, month, day } (month is 1-based). */
+export function parseISODate(s) {
+  const [y, m, d] = s.split('-').map(Number)
+  return { year: y, month: m, day: d }
+}
+
+/** Build a local JS Date at midnight from 'YYYY-MM-DD'. */
+export function toDate(isoDate) {
+  return new Date(isoDate + 'T00:00:00')
+}
+
+/** Add `days` days to an ISO date string, return new ISO string. */
+export function addDays(isoDate, days) {
+  const d = toDate(isoDate)
+  d.setDate(d.getDate() + days)
+  return toISODate(d)
+}
+
+/** Whole-day difference between two ISO date strings (b - a). */
+export function daysBetween(isoA, isoB) {
+  return Math.round((toDate(isoB) - toDate(isoA)) / (24 * 3600 * 1000))
+}
+
+/** Today's date as 'YYYY-MM-DD' (local). */
+export function todayISO() {
+  return toISODate(new Date())
+}
+
+/**
+ * Current UTC timestamp in compact RFC 5545 form (e.g. '20260711T085008Z').
+ * Used for DTSTAMP on new events.
+ */
+export function nowCompactTimestamp() {
+  return new Date().toISOString().replace(/[-:]/g, '').split('.')[0] + 'Z'
+}
+
+/** Format a JS Date's time as 'HH:MM'. */
+export function toHHMM(date) {
+  return `${pad2(date.getHours())}:${pad2(date.getMinutes())}`
+}
diff --git a/src/lib/event-factory.js b/src/lib/event-factory.js
new file mode 100644
index 0000000..0dff148
--- /dev/null
+++ b/src/lib/event-factory.js
@@ -0,0 +1,41 @@
+/**
+ * Event model factory and helpers.
+ *
+ * The EventModel is the plain-JS shape used throughout the app (see
+ * docs/superpowers/specs/... §4.2). This module centralizes creation
+ * and cloning so that useCalendar.js stays focused on state management.
+ */
+import { todayISO, nowCompactTimestamp } from './date.js'
+
+/**
+ * Create a new single-occurrence event with sensible defaults.
+ * @param {object} [overrides] - partial fields to override
+ * @returns {object} a fresh EventModel
+ */
+export function createEvent(overrides = {}) {
+  return {
+    uid: crypto.randomUUID(),
+    summary: '新日程',
+    location: '',
+    description: '',
+    dtstartDate: todayISO(),
+    dtstartTime: '09:00',
+    dtendTime: '10:00',
+    tzid: 'UTC',
+    rrule: null,
+    exdates: [],
+    _raw: { dtstamp: nowCompactTimestamp() },
+    ...overrides,
+  }
+}
+
+/**
+ * Deep-clone an event via structuredClone (available in modern browsers
+ * and Node 17+). Falls back to JSON round-trip for older environments.
+ */
+export function cloneEvent(ev) {
+  if (typeof structuredClone === 'function') {
+    return structuredClone(ev)
+  }
+  return JSON.parse(JSON.stringify(ev))
+}
diff --git a/src/lib/organize.js b/src/lib/organize.js
index 564d1e3..3cd4d76 100644
--- a/src/lib/organize.js
+++ b/src/lib/organize.js
@@ -3,6 +3,7 @@
  * Collapses a flat list of per-occurrence events into RRULE + EXDATE.
  */
 import { bydayFromDate } from './weekday.js'
+import { toDate, toISODate, addDays, daysBetween } from './date.js'
 
 // ------------------------------------------------------------------ //
 // Grouping
@@ -13,11 +14,11 @@ import { bydayFromDate } from './weekday.js'
  * wall-clock time, duration (minutes), and weekday.
  */
 export function seriesKey(ev) {
-  const start = new Date(ev.dtstartDate + 'T' + ev.dtstartTime + ':00')
   const [sh, sm] = ev.dtstartTime.split(':').map(Number)
   const [eh, em] = ev.dtendTime.split(':').map(Number)
   const durationMin = (eh * 60 + em) - (sh * 60 + sm)
-  return [ev.summary, ev.location, ev.dtstartTime, durationMin, start.getDay()].join('|')
+  const weekday = toDate(ev.dtstartDate).getDay()
+  return [ev.summary, ev.location, ev.dtstartTime, durationMin, weekday].join('|')
 }
 
 // ------------------------------------------------------------------ //
@@ -37,9 +38,7 @@ export function detectInterval(dateStrings) {
   const dates = [...dateStrings].sort()
   const gaps = []
   for (let i = 0; i < dates.length - 1; i++) {
-    const a = new Date(dates[i] + 'T00:00:00')
-    const b = new Date(dates[i + 1] + 'T00:00:00')
-    gaps.push(Math.round((b - a) / (24 * 3600 * 1000)))
+    gaps.push(daysBetween(dates[i], dates[i + 1]))
   }
   let g = gaps[0]
   for (let i = 1; i < gaps.length; i++) {
@@ -51,6 +50,25 @@ export function detectInterval(dateStrings) {
 // ------------------------------------------------------------------ //
 // Series fitting
 // ------------------------------------------------------------------ //
+const byDate = (a, b) => (a.dtstartDate < b.dtstartDate ? -1 : a.dtstartDate > b.dtstartDate ? 1 : 0)
+
+/**
+ * Build the expected occurrence grid from first to last at the given interval.
+ * Returns { expected: string[], missing: string[] } relative to present dates.
+ */
+function buildGrid(firstDate, lastDate, interval, present) {
+  const expected = []
+  let cur = firstDate
+  while (cur <= lastDate) {
+    expected.push(cur)
+    cur = addDays(cur, interval)
+  }
+  const expectedSet = new Set(expected)
+  const missing = expected.filter((d) => !present.has(d))
+  const offGrid = [...present].filter((d) => !expectedSet.has(d))
+  return { expected, missing, offGrid }
+}
+
 /**
  * Try to collapse events into one recurring event.
  * Returns { base, rrule, exdates } or null to decline.
@@ -58,10 +76,8 @@ export function detectInterval(dateStrings) {
 export function fitSeries(evs) {
   if (evs.length < 2) return null
 
-  const sorted = [...evs].sort((a, b) =>
-    a.dtstartDate < b.dtstartDate ? -1 : a.dtstartDate > b.dtstartDate ? 1 : 0,
-  )
-  const starts = sorted.map(e => e.dtstartDate)
+  const sorted = [...evs].sort(byDate)
+  const starts = sorted.map((e) => e.dtstartDate)
   const interval = detectInterval(starts)
   if (!interval) return null
 
@@ -69,28 +85,14 @@ export function fitSeries(evs) {
   const last = sorted[sorted.length - 1]
   const present = new Set(starts)
 
-  // Build expected grid first..last step interval days
-  const expected = []
-  const cur = new Date(first.dtstartDate + 'T00:00:00')
-  const lastDate = new Date(last.dtstartDate + 'T00:00:00')
-  while (cur <= lastDate) {
-    const y = cur.getFullYear()
-    const m = String(cur.getMonth() + 1).padStart(2, '0')
-    const d = String(cur.getDate()).padStart(2, '0')
-    expected.push(`${y}-${m}-${d}`)
-    cur.setDate(cur.getDate() + interval)
-  }
-
-  const expectedSet = new Set(expected)
-  const missing = expected.filter(d => !present.has(d))
-  const offGrid = starts.filter(d => !expectedSet.has(d))
+  const { missing, offGrid } = buildGrid(first.dtstartDate, last.dtstartDate, interval, present)
   if (offGrid.length > 0) return null
 
   // "Mostly regular": at least 2 occurrences and occurrences >= skips
   if (evs.length < 2 || missing.length >= evs.length) return null
 
   // Build rrule
-  const firstDate = new Date(first.dtstartDate + 'T00:00:00')
+  const firstDate = toDate(first.dtstartDate)
   let freq, step, byday
   if (interval % 7 === 0) {
     freq = 'WEEKLY'
@@ -127,10 +129,7 @@ export function organize(events) {
   let nFlat = 0
 
   for (const evs of groups.values()) {
-    // Sort by date
-    evs.sort((a, b) =>
-      a.dtstartDate < b.dtstartDate ? -1 : a.dtstartDate > b.dtstartDate ? 1 : 0,
-    )
+    evs.sort(byDate)
 
     const fit = fitSeries(evs)
     if (fit) {
diff --git a/src/lib/recur.js b/src/lib/recur.js
index 7e3c3ae..b29b209 100644
--- a/src/lib/recur.js
+++ b/src/lib/recur.js
@@ -2,6 +2,7 @@
  * Recurrence grid expansion - port of ical_editor.py's occurrence generation.
  * occurrences are computed on-demand from exdates, never stored on the model.
  */
+import { toISODate, toDate, addDays } from './date.js'
 
 /** Interval in days for a recurring event. */
 export function intervalDays(ev) {
@@ -11,12 +12,7 @@ export function intervalDays(ev) {
   return freq === 'WEEKLY' ? 7 * step : step
 }
 
-function toISO(date) {
-  const y = date.getFullYear()
-  const m = String(date.getMonth() + 1).padStart(2, '0')
-  const d = String(date.getDate()).padStart(2, '0')
-  return `${y}-${m}-${d}`
-}
+const MAX_OCCURRENCES = 500
 
 /**
  * Expand a recurring event into an occurrence grid.
@@ -33,20 +29,18 @@ export function expandOccurrences(ev) {
     return [{ date: ev.dtstartDate, skipped: false }]
   }
 
-  const start = new Date(ev.dtstartDate + 'T00:00:00')
   const until = ev.rrule.untilDate
-    ? new Date(ev.rrule.untilDate + 'T00:00:00')
-    : new Date(start.getTime() + 365 * 24 * 3600 * 1000) // default +1 year
+    ? toDate(ev.rrule.untilDate)
+    : new Date(toDate(ev.dtstartDate).getTime() + 365 * 24 * 3600 * 1000)
 
   const exdateSet = new Set(ev.exdates || [])
   const occs = []
-  const cur = new Date(start)
+  let cur = ev.dtstartDate
   let count = 0
 
-  while (cur <= until && count < 500) {
-    const iso = toISO(cur)
-    occs.push({ date: iso, skipped: exdateSet.has(iso) })
-    cur.setDate(cur.getDate() + step)
+  while (cur <= toISODate(until) && count < MAX_OCCURRENCES) {
+    occs.push({ date: cur, skipped: exdateSet.has(cur) })
+    cur = addDays(cur, step)
     count++
   }
 
diff --git a/src/lib/split.js b/src/lib/split.js
new file mode 100644
index 0000000..7d6a6ae
--- /dev/null
+++ b/src/lib/split.js
@@ -0,0 +1,50 @@
+/**
+ * Split a recurring event into two at a given date.
+ *
+ * Extracted from useCalendar.js as a pure function so the store stays
+ * focused on state management. The grid-snapping logic reuses
+ * intervalDays from recur.js (single source of truth for step size).
+ *
+ * @param {object} ev - the event to split (must have a non-null rrule)
+ * @param {string} splitDate - 'YYYY-MM-DD', must be within the event's range
+ * @returns {[object, object]|null} [partA, partB] or null if the date is
+ *   out of range or the event is not recurring.
+ */
+import { intervalDays } from './recur.js'
+import { cloneEvent } from './event-factory.js'
+import { toDate, toISODate, addDays, daysBetween } from './date.js'
+
+/** Snap a date forward to the next grid point on or after it. */
+function snapToGrid(startDate, targetDate, stepDays, untilDate) {
+  const start = toDate(startDate)
+  let snap = toDate(targetDate)
+  if (snap < start) snap = new Date(start)
+  while (daysBetween(startDate, toISODate(snap)) % stepDays !== 0) {
+    snap.setDate(snap.getDate() + 1)
+    if (snap > toDate(untilDate)) return null
+  }
+  return toISODate(snap)
+}
+
+export function splitRecurringEvent(ev, splitDate) {
+  if (!ev.rrule) return null
+  const { untilDate } = ev.rrule
+  if (splitDate <= ev.dtstartDate || splitDate > untilDate) return null
+
+  const step = intervalDays(ev)
+  const snapStr = snapToGrid(ev.dtstartDate, splitDate, step, untilDate)
+  if (!snapStr) return null
+
+  const prevStr = addDays(snapStr, -step)
+
+  const partA = cloneEvent(ev)
+  partA.rrule = { ...ev.rrule, untilDate: prevStr }
+  partA.exdates = ev.exdates.filter((d) => d < splitDate)
+
+  const partB = cloneEvent(ev)
+  partB.uid = crypto.randomUUID()
+  partB.dtstartDate = snapStr
+  partB.exdates = ev.exdates.filter((d) => d >= splitDate)
+
+  return [partA, partB]
+}