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>

frontend/src/components/event_modal.rs

@@ -63,7 +63,7 @@ pub fn EventModal(props: &EventModalProps) -> Html {
                                 html! {
                                     <div class="event-detail">
                                         <strong>{"End:"}</strong>
-                                        <span>{format_datetime(end, event.all_day)}</span>
+                                        <span>{format_datetime_end(end, event.all_day)}</span>
                                     </div>
                                 }
                             } else {
@@ -221,6 +221,17 @@ fn format_datetime(dt: &DateTime<Utc>, all_day: bool) -> String {
     }
 }
 
+fn format_datetime_end(dt: &DateTime<Utc>, all_day: bool) -> String {
+    if all_day {
+        // For all-day events, subtract one day from end date for display
+        // RFC-5545 uses exclusive end dates, but users expect inclusive display
+        let display_date = *dt - chrono::Duration::days(1);
+        display_date.format("%B %d, %Y").to_string()
+    } else {
+        dt.format("%B %d, %Y at %I:%M %p").to_string()
+    }
+}
+
 fn format_recurrence_rule(rrule: &str) -> String {
     // Basic parsing of RRULE to display user-friendly text
     if rrule.contains("FREQ=DAILY") {

frontend/src/services/calendar_service.rs

@@ -439,9 +439,17 @@ impl CalendarService {
                     let mut occurrence_event = base_event.clone();
                     occurrence_event.dtstart = occurrence_datetime;
                     occurrence_event.dtstamp = chrono::Utc::now(); // Update DTSTAMP for each occurrence
+                    
 
-                    if let Some(end) = base_event.dtend {
-                        occurrence_event.dtend = Some(end + Duration::days(days_diff));
+                    if let Some(base_end) = base_event.dtend {
+                        if base_event.all_day {
+                            // For all-day events, maintain the RFC-5545 end date pattern
+                            // End date should always be exactly one day after start date
+                            occurrence_event.dtend = Some(occurrence_datetime + Duration::days(1));
+                        } else {
+                            // For timed events, preserve the original duration
+                            occurrence_event.dtend = Some(base_end + Duration::days(days_diff));
+                        }
                     }
 
                     occurrences.push(occurrence_event);