diff --git a/apps/pwa/src/lib/eventDateTime.ts b/apps/pwa/src/lib/eventDateTime.ts
index d499be3..5122299 100644
--- a/apps/pwa/src/lib/eventDateTime.ts
+++ b/apps/pwa/src/lib/eventDateTime.ts
@@ -59,3 +59,91 @@ export function serializeEventDateTime(
 export function localWallClockToUtcIso(date: string, time: string): string {
   return new Date(`${date}T${time}:00`).toISOString()
 }
+
+// ─── Private date helpers (local accessor pattern — WR-05 constraint) ────────
+// NEVER use toISOString().slice(0,10) — that returns the UTC date, not the
+// local date. Always use getFullYear/getMonth/getDate/getHours/getMinutes.
+
+function pad2(n: number): string {
+  return n < 10 ? `0${n}` : `${n}`
+}
+
+/** Format a Date as 'YYYY-MM-DD' using LOCAL calendar accessors. */
+function localDateISO(d: Date): string {
+  return `${d.getFullYear()}-${pad2(d.getMonth() + 1)}-${pad2(d.getDate())}`
+}
+
+/** Format a Date as 'HH:MM' using LOCAL clock accessors. */
+function localTimeHHMM(d: Date): string {
+  return `${pad2(d.getHours())}:${pad2(d.getMinutes())}`
+}
+
+/**
+ * Return the number of whole calendar days between two 'YYYY-MM-DD' strings.
+ * Positive when end > start, negative when end < start.
+ * Uses midnight-local Date arithmetic to stay in the local calendar.
+ */
+function dateDiffDays(startDate: string, endDate: string): number {
+  const startMs = new Date(`${startDate}T00:00:00`).getTime()
+  const endMs = new Date(`${endDate}T00:00:00`).getTime()
+  return Math.round((endMs - startMs) / (24 * 60 * 60 * 1000))
+}
+
+/**
+ * Add a whole number of days to a 'YYYY-MM-DD' string, returning a new
+ * 'YYYY-MM-DD' string. Uses local midnight to avoid DST-boundary issues.
+ */
+function addDaysISO(dateStr: string, days: number): string {
+  const d = new Date(`${dateStr}T00:00:00`)
+  d.setDate(d.getDate() + days)
+  return localDateISO(d)
+}
+
+// ─── D-04: Duration-preservation helpers ─────────────────────────────────────
+
+/**
+ * Compute a new timed event end when the start changes, preserving duration.
+ *
+ * D-04 contract:
+ *   - newEnd = newStart + (oldEnd − oldStart)
+ *   - Floor: if oldEnd ≤ oldStart (stale/invalid), newEnd = newStart + 1h
+ *
+ * Returns { endDate: 'YYYY-MM-DD', endTime: 'HH:MM' } using LOCAL accessors
+ * (WR-05: never UTC-sliced).
+ */
+export function computeNewTimedEnd(
+  newStartDate: string,
+  newStartTime: string,
+  oldStartDate: string,
+  oldStartTime: string,
+  oldEndDate: string,
+  oldEndTime: string,
+): { endDate: string; endTime: string } {
+  const oldStartMs = new Date(`${oldStartDate}T${oldStartTime}:00`).getTime()
+  const oldEndMs = new Date(`${oldEndDate}T${oldEndTime}:00`).getTime()
+  const deltaMs = oldEndMs > oldStartMs ? oldEndMs - oldStartMs : 60 * 60 * 1000 // 1h floor
+  const newEndDate = new Date(new Date(`${newStartDate}T${newStartTime}:00`).getTime() + deltaMs)
+  return {
+    endDate: localDateISO(newEndDate),
+    endTime: localTimeHHMM(newEndDate),
+  }
+}
+
+/**
+ * Compute a new all-day event end when the start date changes, preserving
+ * the inclusive day-span.
+ *
+ * D-04 contract:
+ *   - span = max(0, oldEnd − oldStart) days  (Math.max floors negative spans to 0)
+ *   - newEnd = newStart + span
+ *
+ * Returns 'YYYY-MM-DD' (inclusive end date).
+ */
+export function computeNewAllDayEnd(
+  newStartDate: string,
+  oldStartDate: string,
+  oldEndDate: string,
+): string {
+  const span = Math.max(0, dateDiffDays(oldStartDate, oldEndDate))
+  return addDaysISO(newStartDate, span)
+}