src/utils/date.js

@@ -99,12 +99,30 @@ function getWeeklyOccurrenceIndex(event, dateStr) {
 
   // For same week as base start, count from base start to target
   if (currentBlockStart.getTime() === baseBlockStart.getTime()) {
+    // Special handling for the first week - only count occurrences on or after base date
+    if (d.getTime() === baseStart.getTime()) {
+      return 0 // Base occurrence is always index 0
+    }
+    
+    if (d < baseStart) {
+      return null // Dates before base start in same week are not valid occurrences
+    }
+    
     let occurrenceIndex = 0
     const cursor = new Date(baseStart)
-    while (cursor < d) {
+    
+    // Count the base occurrence first
+    if (pattern[cursor.getDay()]) occurrenceIndex++
+    
+    // Move to the next day and count until we reach the target
+    cursor.setDate(cursor.getDate() + 1)
+    while (cursor <= d) {
       if (pattern[cursor.getDay()]) occurrenceIndex++
       cursor.setDate(cursor.getDate() + 1)
     }
+    
+    // Subtract 1 because we want the index, not the count
+    occurrenceIndex--
 
     // Check against repeat count limit
     if (event.repeatCount !== 'unlimited') {
@@ -116,9 +134,23 @@ function getWeeklyOccurrenceIndex(event, dateStr) {
   }
 
   // For different weeks, calculate based on complete intervals
-  const weekdaysPerInterval = pattern.filter(Boolean).length
+  // Calculate how many pattern days actually occur in the first week (from base start onward)
+  let firstWeekPatternDays = 0
+  const firstWeekCursor = new Date(baseStart)
+  const firstWeekEnd = new Date(baseBlockStart)
+  firstWeekEnd.setDate(firstWeekEnd.getDate() + 6) // End of first week (Sunday)
+  
+  while (firstWeekCursor <= firstWeekEnd) {
+    if (pattern[firstWeekCursor.getDay()]) firstWeekPatternDays++
+    firstWeekCursor.setDate(firstWeekCursor.getDate() + 1)
+  }
+  
+  // For subsequent complete intervals, use the full pattern count
+  const fullWeekdaysPerInterval = pattern.filter(Boolean).length
   const completeIntervals = blocksDiff / interval
-  let occurrenceIndex = completeIntervals * weekdaysPerInterval
+  
+  // First interval uses actual first week count, remaining intervals use full count
+  let occurrenceIndex = firstWeekPatternDays + (completeIntervals - 1) * fullWeekdaysPerInterval
 
   // Add occurrences from the current week up to the target date
   const cursor = new Date(currentBlockStart)