Cleanup date and recurrence calculations.

src/utils/date.js

@@ -139,6 +139,68 @@ function getOccurrenceIndex(event, dateStr, timeZone = DEFAULT_TZ) {
   return null
 }
 
+// Reverse lookup: given a recurrence index (0-based) return the occurrence start date string.
+// Returns null if the index is out of range or the event is not repeating.
+function getWeeklyOccurrenceDate(event, occurrenceIndex, timeZone = DEFAULT_TZ) {
+  if (!event?.isRepeating || event.repeat !== 'weeks') return null
+  if (occurrenceIndex < 0 || !Number.isInteger(occurrenceIndex)) return null
+  if (event.repeatCount !== 'unlimited' && occurrenceIndex >= event.repeatCount) return null
+  const pattern = event.repeatWeekdays || []
+  if (!pattern.some(Boolean)) return null
+  const interval = event.repeatInterval || 1
+  const baseStart = fromLocalString(event.startDate, timeZone)
+  if (occurrenceIndex === 0) return toLocalString(baseStart, timeZone)
+  const baseWeekMonday = getMondayOfISOWeek(baseStart, timeZone)
+  const baseDow = dateFns.getDay(baseStart)
+  // Sorted list of active weekday indices
+  const patternDays = []
+  for (let d = 0; d < 7; d++) if (pattern[d]) patternDays.push(d)
+  // First (possibly partial) week: only pattern days >= baseDow and >= baseStart date
+  const firstWeekDates = []
+  for (const d of patternDays) {
+    if (d < baseDow) continue
+    const date = dateFns.addDays(baseWeekMonday, d)
+    if (date < baseStart) continue
+    firstWeekDates.push(date)
+  }
+  const F = firstWeekDates.length
+  if (occurrenceIndex < F) {
+    return toLocalString(firstWeekDates[occurrenceIndex], timeZone)
+  }
+  const remaining = occurrenceIndex - F
+  const P = patternDays.length
+  if (P === 0) return null
+  // Determine aligned week group (k >= 1) in which the remaining-th occurrence lies
+  const k = Math.floor(remaining / P) + 1 // 1-based aligned week count after base week
+  const indexInWeek = remaining % P
+  const dow = patternDays[indexInWeek]
+  const occurrenceDate = dateFns.addDays(baseWeekMonday, k * interval * 7 + dow)
+  return toLocalString(occurrenceDate, timeZone)
+}
+
+function getMonthlyOccurrenceDate(event, occurrenceIndex, timeZone = DEFAULT_TZ) {
+  if (!event?.isRepeating || event.repeat !== 'months') return null
+  if (occurrenceIndex < 0 || !Number.isInteger(occurrenceIndex)) return null
+  if (event.repeatCount !== 'unlimited' && occurrenceIndex >= event.repeatCount) return null
+  const interval = event.repeatInterval || 1
+  const baseStart = fromLocalString(event.startDate, timeZone)
+  const targetMonthOffset = occurrenceIndex * interval
+  const monthDate = dateFns.addMonths(baseStart, targetMonthOffset)
+  // Adjust day for shorter months (clamp like forward logic)
+  const baseDay = dateFns.getDate(baseStart)
+  const daysInTargetMonth = dateFns.getDaysInMonth(monthDate)
+  const day = Math.min(baseDay, daysInTargetMonth)
+  const actual = makeTZDate(dateFns.getYear(monthDate), dateFns.getMonth(monthDate), day, timeZone)
+  return toLocalString(actual, timeZone)
+}
+
+function getOccurrenceDate(event, occurrenceIndex, timeZone = DEFAULT_TZ) {
+  if (!event?.isRepeating || event.repeat === 'none') return null
+  if (event.repeat === 'weeks') return getWeeklyOccurrenceDate(event, occurrenceIndex, timeZone)
+  if (event.repeat === 'months') return getMonthlyOccurrenceDate(event, occurrenceIndex, timeZone)
+  return null
+}
+
 function getVirtualOccurrenceEndDate(event, occurrenceStartDate, timeZone = DEFAULT_TZ) {
   const baseStart = fromLocalString(event.startDate, timeZone)
   const baseEnd = fromLocalString(event.endDate, timeZone)
@@ -237,6 +299,7 @@ export {
   getMondayOfISOWeek,
   mondayIndex,
   getOccurrenceIndex,
+  getOccurrenceDate,
   getVirtualOccurrenceEndDate,
   // formatting & localization
   pad,