diff --git a/apps/pwa/src/components/EventForm.tsx b/apps/pwa/src/components/EventForm.tsx
index 706b0c4..5635fc9 100644
--- a/apps/pwa/src/components/EventForm.tsx
+++ b/apps/pwa/src/components/EventForm.tsx
@@ -74,6 +74,27 @@ function writeLastCalendarUrl(url: string): void {
   }
 }
 
+/**
+ * CR-03: convert an EXCLUSIVE all-day end date ('YYYY-MM-DD') to the INCLUSIVE
+ * last day the form displays. The occurrence/expand contract keeps all-day ends
+ * exclusive (matching the DTEND Fastmail stores, RFC-5545 §3.6.1), and
+ * buildVeventString re-advances the inclusive form value by one day on write.
+ * Without this subtraction, round-tripping an edit re-advances an already-exclusive
+ * end, silently growing multi-day all-day events by one day per save. Parses by
+ * UTC components so the roll-back is DST-safe (mirrors vevent.ts's roll-forward).
+ */
+function exclusiveEndToInclusiveDate(dateStr: string): string {
+  const m = /^(\d{4})-(\d{2})-(\d{2})$/.exec(dateStr)
+  if (!m) return dateStr
+  const [, y, mo, d] = m
+  const date = new Date(Date.UTC(Number(y), Number(mo) - 1, Number(d)))
+  date.setUTCDate(date.getUTCDate() - 1)
+  const yy = String(date.getUTCFullYear())
+  const mm = String(date.getUTCMonth() + 1).padStart(2, '0')
+  const dd = String(date.getUTCDate()).padStart(2, '0')
+  return `${yy}-${mm}-${dd}`
+}
+
 /**
  * Parse an ISO date string (possibly with time + offset) into
  * { date: 'YYYY-MM-DD', time: 'HH:MM' }. Falls back to today/09:00 if malformed.
@@ -143,12 +164,17 @@ export function EventForm() {
 
   const initStart = occurrence ? parseDateTime(occurrence.start) : { date: todayIso(), time: '09:00' }
   const initEnd = occurrence ? parseDateTime(occurrence.end) : { date: todayIso(), time: '10:00' }
+  // CR-03: occurrence.end for an all-day event is the EXCLUSIVE DTEND; the form's
+  // end-date input is the INCLUSIVE last day. Convert when pre-filling so a re-edit
+  // does not re-advance the span (buildVeventString rolls forward again on write).
+  const initEndDate =
+    occurrence?.allDay ? exclusiveEndToInclusiveDate(initEnd.date) : initEnd.date
 
   const [title, setTitle] = useState(occurrence?.title ?? '')
   const [allDay, setAllDay] = useState(occurrence?.allDay ?? false)
   const [startDate, setStartDate] = useState(initStart.date)
   const [startTime, setStartTime] = useState(initStart.time)
-  const [endDate, setEndDate] = useState(initEnd.date)
+  const [endDate, setEndDate] = useState(initEndDate)
   const [endTime, setEndTime] = useState(initEnd.time)
   const [recurrence, setRecurrence] = useState<RecurrencePreset>('none')
   const [location, setLocation] = useState(occurrence?.location ?? '')
@@ -179,11 +205,15 @@ export function EventForm() {
     if (eventFormOpen) {
       const startParsed = occurrence ? parseDateTime(occurrence.start) : { date: todayIso(), time: '09:00' }
       const endParsed = occurrence ? parseDateTime(occurrence.end) : { date: todayIso(), time: '10:00' }
+      // CR-03: see exclusiveEndToInclusiveDate — pre-fill the inclusive last day for
+      // all-day events so re-saving an edit does not grow the span by a day each time.
+      const endDateValue =
+        occurrence?.allDay ? exclusiveEndToInclusiveDate(endParsed.date) : endParsed.date
       setTitle(occurrence?.title ?? '')
       setAllDay(occurrence?.allDay ?? false)
       setStartDate(startParsed.date)
       setStartTime(startParsed.time)
-      setEndDate(endParsed.date)
+      setEndDate(endDateValue)
       setEndTime(endParsed.time)
       // WR-03 recurrence: derive from occurrence if present; default 'none' only when
       // genuinely absent. Note: occurrence.recurrence is not in CalendarOccurrence type