Fix all-day recurring events RFC-5545 compliance

- Set all_day flag properly when creating VEvent in series handler
- Improve all-day event detection using VALUE=DATE parameter
- Add RFC-5545 compliance for exclusive end dates (backend adds 1 day)
- Fix end date display in event modal (frontend subtracts 1 day for display)
- Fix recurring all-day event expansion to maintain proper end date pattern

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

backend/src/calendar.rs

@@ -361,11 +361,12 @@ impl CalDAVClient {
             None
         };
 
-        // Determine if it's an all-day event
-        let all_day = properties
-            .get("DTSTART")
-            .map(|s| !s.contains("T"))
-            .unwrap_or(false);
+        // Determine if it's an all-day event by checking for VALUE=DATE parameter
+        let empty_string = String::new();
+        let dtstart_raw = properties.get("DTSTART").unwrap_or(&empty_string);
+        let all_day = dtstart_raw.contains("VALUE=DATE") || (!dtstart_raw.contains("T") && dtstart_raw.len() == 8);
+        
+        eprintln!("🔍 DTSTART parsing: '{}' -> all_day: {}", dtstart_raw, all_day);
 
         // Parse status
         let status = properties

backend/src/handlers/events.rs

@@ -458,9 +458,15 @@ pub async fn create_event(
         parse_event_datetime(&request.start_date, &request.start_time, request.all_day)
             .map_err(|e| ApiError::BadRequest(format!("Invalid start date/time: {}", e)))?;
 
-    let end_datetime = parse_event_datetime(&request.end_date, &request.end_time, request.all_day)
+    let mut end_datetime = parse_event_datetime(&request.end_date, &request.end_time, request.all_day)
         .map_err(|e| ApiError::BadRequest(format!("Invalid end date/time: {}", e)))?;
 
+    // For all-day events, add one day to end date for RFC-5545 compliance
+    // RFC-5545 uses exclusive end dates for all-day events
+    if request.all_day {
+        end_datetime = end_datetime + chrono::Duration::days(1);
+    }
+
     // Validate that end is after start (allow equal times for all-day events)
     if request.all_day {
         if end_datetime < start_datetime {
@@ -756,9 +762,15 @@ pub async fn update_event(
         parse_event_datetime(&request.start_date, &request.start_time, request.all_day)
             .map_err(|e| ApiError::BadRequest(format!("Invalid start date/time: {}", e)))?;
 
-    let end_datetime = parse_event_datetime(&request.end_date, &request.end_time, request.all_day)
+    let mut end_datetime = parse_event_datetime(&request.end_date, &request.end_time, request.all_day)
         .map_err(|e| ApiError::BadRequest(format!("Invalid end date/time: {}", e)))?;
 
+    // For all-day events, add one day to end date for RFC-5545 compliance
+    // RFC-5545 uses exclusive end dates for all-day events
+    if request.all_day {
+        end_datetime = end_datetime + chrono::Duration::days(1);
+    }
+
     // Validate that end is after start (allow equal times for all-day events)
     if request.all_day {
         if end_datetime < start_datetime {

backend/src/handlers/series.rs

@@ -175,6 +175,7 @@ pub async fn create_event_series(
     // Create the VEvent for the series
     let mut event = VEvent::new(uid.clone(), start_datetime);
     event.dtend = Some(end_datetime);
+    event.all_day = request.all_day; // Set the all_day flag properly
     event.summary = if request.title.trim().is_empty() {
         None
     } else {