Fix all-day event date display bug: events created 9/4-9/6 now show correctly instead of 9/3-9/5

- Backend: Store all-day events at noon UTC instead of midnight to avoid timezone boundary issues
- Backend: Remove local timezone conversion for all-day events in series handler
- Frontend: Skip timezone conversion when extracting dates from all-day events for display
- Frontend: Extract dates directly from UTC for all-day events in event_spans_date function

The issue was that timezone conversion of UTC midnight could shift dates backward in western timezones.
Now all-day events use noon UTC storage and pure date extraction without timezone conversion.

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

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

backend/src/handlers/events.rs

@@ -852,10 +852,11 @@ fn parse_event_datetime(
         .map_err(|_| format!("Invalid date format: {}. Expected YYYY-MM-DD", date_str))?;
 
     if all_day {
-        // For all-day events, use midnight UTC
+        // For all-day events, use noon UTC to avoid timezone boundary issues
+        // This ensures the date remains correct when converted to any local timezone
         let datetime = date
-            .and_hms_opt(0, 0, 0)
+            .and_hms_opt(12, 0, 0)
-            .ok_or_else(|| "Failed to create midnight datetime".to_string())?;
+            .ok_or_else(|| "Failed to create noon datetime".to_string())?;
         Ok(Utc.from_utc_datetime(&datetime))
     } else {
         // Parse the time

backend/src/handlers/series.rs

@@ -397,8 +397,9 @@ pub async fn update_event_series(
     };
 
     let (start_datetime, end_datetime) = if request.all_day {
+        // For all-day events, use noon UTC to avoid timezone boundary issues
         let start_dt = start_date
-            .and_hms_opt(0, 0, 0)
+            .and_hms_opt(12, 0, 0)
             .ok_or_else(|| ApiError::BadRequest("Invalid start date".to_string()))?;
 
         // For all-day events, also preserve the original date pattern
@@ -414,20 +415,13 @@ pub async fn update_event_series(
         };
 
         let end_dt = end_date
-            .and_hms_opt(23, 59, 59)
+            .and_hms_opt(12, 0, 0)
             .ok_or_else(|| ApiError::BadRequest("Invalid end date".to_string()))?;
 
-        // Convert from local time to UTC
+        // For all-day events, use UTC directly (no local conversion needed)
-        let start_local = chrono::Local.from_local_datetime(&start_dt)
-            .single()
-            .ok_or_else(|| ApiError::BadRequest("Ambiguous start datetime".to_string()))?;
-        let end_local = chrono::Local.from_local_datetime(&end_dt)
-            .single()
-            .ok_or_else(|| ApiError::BadRequest("Ambiguous end datetime".to_string()))?;
-        
         (
-            start_local.with_timezone(&chrono::Utc),
+            chrono::Utc.from_utc_datetime(&start_dt),
-            end_local.with_timezone(&chrono::Utc),
+            chrono::Utc.from_utc_datetime(&end_dt),
         )
     } else {
         let start_time = if !request.start_time.is_empty() {

frontend/src/components/week_view.rs

@@ -1320,11 +1320,23 @@ fn calculate_event_layout(events: &[VEvent], date: NaiveDate, time_increment: u3
 
 // Check if an all-day event spans the given date
 fn event_spans_date(event: &VEvent, date: NaiveDate) -> bool {
-    let start_date = event.dtstart.with_timezone(&Local).date_naive();
+    let start_date = if event.all_day {
+        // For all-day events, extract date directly from UTC without timezone conversion
+        // since all-day events are stored at noon UTC to avoid timezone boundary issues
+        event.dtstart.date_naive()
+    } else {
+        event.dtstart.with_timezone(&Local).date_naive()
+    };
+    
     let end_date = if let Some(dtend) = event.dtend {
-        // For all-day events, dtend is often set to the day after the last day
+        if event.all_day {
-        // So we need to subtract a day to get the actual last day of the event
+            // For all-day events, dtend is set to the day after the last day (RFC 5545)
-        dtend.with_timezone(&Local).date_naive() - chrono::Duration::days(1)
+            // Extract date directly from UTC and subtract a day to get actual last day
+            dtend.date_naive() - chrono::Duration::days(1)
+        } else {
+            // For timed events, use timezone conversion
+            dtend.with_timezone(&Local).date_naive()
+        }
     } else {
         // Single day event
         start_date