src/utils/date.js

@@ -100,9 +100,13 @@ function getWeeklyOccurrenceIndex(event, dateStr, timeZone = DEFAULT_TZ) {
   const weekDiff = dateFns.differenceInCalendarWeeks(currentBlockStart, baseBlockStart)
   if (weekDiff < 0 || weekDiff % interval !== 0) return null
 
+  const baseDow = dateFns.getDay(baseStart)
+  const baseCountsAsPattern = !!pattern[baseDow]
+
   // Same ISO week as base: count pattern days from baseStart up to target (inclusive)
   if (weekDiff === 0) {
-    const n = countPatternDaysInInterval(baseStart, target, pattern) - 1
+    let n = countPatternDaysInInterval(baseStart, target, pattern) - 1
+    if (!baseCountsAsPattern) n += 1 // Shift indices so base occurrence stays 0
     return n < 0 || n >= event.repeatCount ? null : n
   }
 
@@ -114,7 +118,8 @@ function getWeeklyOccurrenceIndex(event, dateStr, timeZone = DEFAULT_TZ) {
   const middleWeeksCount = alignedWeeksBetween > 0 ? alignedWeeksBetween * fullPatternWeekCount : 0
   // Count pattern days in the current (possibly partial) week from currentBlockStart..target
   const currentWeekCount = countPatternDaysInInterval(currentBlockStart, target, pattern)
-  const n = firstWeekCount + middleWeeksCount + currentWeekCount - 1
+  let n = firstWeekCount + middleWeeksCount + currentWeekCount - 1
+  if (!baseCountsAsPattern) n += 1
   return n >= event.repeatCount ? null : n
 }
 
@@ -155,6 +160,11 @@ function getWeeklyOccurrenceDate(event, occurrenceIndex, timeZone = DEFAULT_TZ)
   if (occurrenceIndex === 0) return toLocalString(baseStart, timeZone)
   const baseWeekMonday = getMondayOfISOWeek(baseStart, timeZone)
   const baseDow = dateFns.getDay(baseStart)
+  const baseCountsAsPattern = !!pattern[baseDow]
+  // Adjust index if base weekday is not part of the pattern (pattern occurrences shift by +1)
+  let occ = occurrenceIndex
+  if (!baseCountsAsPattern) occ -= 1
+  if (occ < 0) return null
   // Sorted list of active weekday indices
   const patternDays = []
   for (let d = 0; d < 7; d++) if (pattern[d]) patternDays.push(d)
@@ -167,10 +177,10 @@ function getWeeklyOccurrenceDate(event, occurrenceIndex, timeZone = DEFAULT_TZ)
     firstWeekDates.push(date)
   }
   const F = firstWeekDates.length
-  if (occurrenceIndex < F) {
-    return toLocalString(firstWeekDates[occurrenceIndex], timeZone)
+  if (occ < F) {
+    return toLocalString(firstWeekDates[occ], timeZone)
   }
-  const remaining = occurrenceIndex - F
+  const remaining = occ - F
   const P = patternDays.length
   if (P === 0) return null
   // Determine aligned week group (k >= 1) in which the remaining-th occurrence lies