fix: respect TZID when updating CalDAV event start/end times

When updating Apple/Synology events that have DTSTART with a TZID
parameter, convert the incoming UTC dateTime to local time in that
timezone before writing back. Prevents ICAL.js from storing UTC
values under a local timezone, which caused +1h shifts on update.

v1.63.4

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "my-weekly-todo-list",
-  "version": "1.63.3",
+  "version": "1.63.4",
   "description": "A web-based weekly task management application that organizes to-dos and calendar events in a single, intuitive weekly view",
   "main": "index.js",
   "scripts": {

src/lib/apple-calendar.ts

@@ -697,25 +697,60 @@ export const updateEvent = async (
       }
     }
 
+    // Check if existing event uses TZID (for recurring events with DST-safe times)
+    const existingDtStart = vevent.getFirstProperty('dtstart');
+    const existingTzid = existingDtStart?.getParameter('tzid') as string | undefined;
+
     if (eventData.start) {
       if (eventData.start.date) {
         event.startDate = ICAL.Time.fromJSDate(new Date(eventData.start.date), true);
         event.startDate.isDate = true;
       } else if (eventData.start.dateTime) {
+        if (existingTzid) {
+          // Convert UTC dateTime to local time in the TZID
+          const d = new Date(eventData.start.dateTime);
+          const parts = new Intl.DateTimeFormat('en-CA', {
+            timeZone: existingTzid, year: 'numeric', month: '2-digit', day: '2-digit',
+            hour: '2-digit', minute: '2-digit', second: '2-digit', hour12: false,
+          }).formatToParts(d);
+          const get = (t: string) => parts.find(p => p.type === t)?.value || '00';
+          const localTime = ICAL.Time.fromData({
+            year: parseInt(get('year')), month: parseInt(get('month')), day: parseInt(get('day')),
+            hour: parseInt(get('hour')), minute: parseInt(get('minute')), second: parseInt(get('second')),
+            isDate: false,
+          });
+          event.startDate = localTime;
+        } else {
           event.startDate = ICAL.Time.fromJSDate(new Date(eventData.start.dateTime), true);
           event.startDate.isDate = false;
         }
       }
+    }
 
     if (eventData.end) {
       if (eventData.end.date) {
         event.endDate = ICAL.Time.fromJSDate(new Date(eventData.end.date), true);
         event.endDate.isDate = true;
       } else if (eventData.end.dateTime) {
+        if (existingTzid) {
+          const d = new Date(eventData.end.dateTime);
+          const parts = new Intl.DateTimeFormat('en-CA', {
+            timeZone: existingTzid, year: 'numeric', month: '2-digit', day: '2-digit',
+            hour: '2-digit', minute: '2-digit', second: '2-digit', hour12: false,
+          }).formatToParts(d);
+          const get = (t: string) => parts.find(p => p.type === t)?.value || '00';
+          const localTime = ICAL.Time.fromData({
+            year: parseInt(get('year')), month: parseInt(get('month')), day: parseInt(get('day')),
+            hour: parseInt(get('hour')), minute: parseInt(get('minute')), second: parseInt(get('second')),
+            isDate: false,
+          });
+          event.endDate = localTime;
+        } else {
           event.endDate = ICAL.Time.fromJSDate(new Date(eventData.end.dateTime), true);
           event.endDate.isDate = false;
         }
       }
+    }
 
     // Update reminders (VALARM)
     if (eventData.reminders !== undefined) {

src/lib/synology-calendar.ts

@@ -714,25 +714,51 @@ export const updateEvent = async (
       }
     }
 
+    // Check if existing event uses TZID (for recurring events with DST-safe times)
+    const existingDtStart = vevent.getFirstProperty('dtstart');
+    const existingTzid = existingDtStart?.getParameter('tzid') as string | undefined;
+
+    const toLocalIcalTime = (isoStr: string, tzid: string) => {
+      const d = new Date(isoStr);
+      const parts = new Intl.DateTimeFormat('en-CA', {
+        timeZone: tzid, year: 'numeric', month: '2-digit', day: '2-digit',
+        hour: '2-digit', minute: '2-digit', second: '2-digit', hour12: false,
+      }).formatToParts(d);
+      const get = (t: string) => parts.find(p => p.type === t)?.value || '00';
+      return ICAL.Time.fromData({
+        year: parseInt(get('year')), month: parseInt(get('month')), day: parseInt(get('day')),
+        hour: parseInt(get('hour')), minute: parseInt(get('minute')), second: parseInt(get('second')),
+        isDate: false,
+      });
+    };
+
     if (eventData.start) {
       if (eventData.start.date) {
         event.startDate = ICAL.Time.fromJSDate(new Date(eventData.start.date), true);
         event.startDate.isDate = true;
       } else if (eventData.start.dateTime) {
+        if (existingTzid) {
+          event.startDate = toLocalIcalTime(eventData.start.dateTime, existingTzid);
+        } else {
           event.startDate = ICAL.Time.fromJSDate(new Date(eventData.start.dateTime), true);
           event.startDate.isDate = false;
         }
       }
+    }
 
     if (eventData.end) {
       if (eventData.end.date) {
         event.endDate = ICAL.Time.fromJSDate(new Date(eventData.end.date), true);
         event.endDate.isDate = true;
       } else if (eventData.end.dateTime) {
+        if (existingTzid) {
+          event.endDate = toLocalIcalTime(eventData.end.dateTime, existingTzid);
+        } else {
           event.endDate = ICAL.Time.fromJSDate(new Date(eventData.end.dateTime), true);
           event.endDate.isDate = false;
         }
       }
+    }
 
     // Update reminders (VALARM)
     if (eventData.reminders !== undefined) {