diff --git a/components/calendar/calendar-agenda-view.tsx b/components/calendar/calendar-agenda-view.tsx
index e8d1c391..15712ff1 100644
--- a/components/calendar/calendar-agenda-view.tsx
+++ b/components/calendar/calendar-agenda-view.tsx
@@ -49,7 +49,7 @@ export function CalendarAgendaView({
 
   const grouped = useMemo(() => {
     const sorted = [...events].sort((a, b) =>
-      new Date(a.start).getTime() - new Date(b.start).getTime()
+      parseISO(a.start).getTime() - parseISO(b.start).getTime()
     );
 
     const groups: DayGroup[] = [];
diff --git a/components/calendar/calendar-month-view.tsx b/components/calendar/calendar-month-view.tsx
index be21714c..dfccf673 100644
--- a/components/calendar/calendar-month-view.tsx
+++ b/components/calendar/calendar-month-view.tsx
@@ -121,13 +121,14 @@ export function CalendarMonthView({
     try {
       const data = JSON.parse(json);
       const originalStart = parseISO(data.originalStart);
+      const event = useCalendarStore.getState().events.find(e => e.id === data.eventId);
+      const isAllDay = event?.showWithoutTime;
       const newStart = new Date(day);
       newStart.setHours(originalStart.getHours(), originalStart.getMinutes(), originalStart.getSeconds(), 0);
-      const newStartISO = format(newStart, "yyyy-MM-dd'T'HH:mm:ss");
+      const newStartISO = isAllDay ? format(newStart, "yyyy-MM-dd") : format(newStart, "yyyy-MM-dd'T'HH:mm:ss");
       if (newStartISO === data.originalStart) return;
       const client = useAuthStore.getState().client;
       if (!client) return;
-      const event = useCalendarStore.getState().events.find(e => e.id === data.eventId);
       const hasParticipants = event?.participants && Object.keys(event.participants).length > 0;
       await useCalendarStore.getState().updateEvent(client, data.eventId, { start: newStartISO }, hasParticipants || undefined);
     } catch {
diff --git a/components/calendar/event-modal.tsx b/components/calendar/event-modal.tsx
index a45e359e..2751ccac 100644
--- a/components/calendar/event-modal.tsx
+++ b/components/calendar/event-modal.tsx
@@ -262,10 +262,10 @@ export function EventModal({
     if (trimmedTitle.length > 500 || description.trim().length > 10000 || location.trim().length > 500) return;
 
     const startStr = allDay
-      ? `${startDate}T00:00:00`
+      ? startDate
       : `${startDate}T${startTime}:00`;
 
-    const start = new Date(startStr);
+    const start = allDay ? parseISO(startStr) : new Date(startStr);
     let duration: string;
 
     if (allDay) {
@@ -393,7 +393,7 @@ export function EventModal({
       uid: newUid,
       title: event.title,
       description: event.description,
-      start: format(newStart, "yyyy-MM-dd'T'HH:mm:ss"),
+      start: event.showWithoutTime ? format(newStart, "yyyy-MM-dd") : format(newStart, "yyyy-MM-dd'T'HH:mm:ss"),
       duration: event.duration,
       timeZone: event.timeZone,
       showWithoutTime: event.showWithoutTime,
diff --git a/components/calendar/mini-calendar.tsx b/components/calendar/mini-calendar.tsx
index 47578533..90ed0898 100644
--- a/components/calendar/mini-calendar.tsx
+++ b/components/calendar/mini-calendar.tsx
@@ -7,7 +7,7 @@ import {
   startOfMonth, endOfMonth, startOfWeek, endOfWeek,
   addMonths, subMonths, addYears, subYears, setMonth, setYear,
   eachDayOfInterval, getMonth, getYear, getISOWeek, getWeek,
-  isSameDay, isSameMonth, isToday, format,
+  isSameDay, isSameMonth, isToday, format, parseISO,
 } from "date-fns";
 import { cn } from "@/lib/utils";
 import type { CalendarEvent } from "@/lib/jmap/types";
@@ -54,7 +54,7 @@ export function MiniCalendar({
   const eventDates = useMemo(() => {
     const set = new Set<string>();
     events.forEach(e => {
-      try { set.add(format(new Date(e.start), "yyyy-MM-dd")); } catch { /* skip */ }
+      try { set.add(format(parseISO(e.start), "yyyy-MM-dd")); } catch { /* skip */ }
     });
     return set;
   }, [events]);
diff --git a/components/calendar/task-modal.tsx b/components/calendar/task-modal.tsx
index 683db859..bf4c11e0 100644
--- a/components/calendar/task-modal.tsx
+++ b/components/calendar/task-modal.tsx
@@ -97,7 +97,7 @@ export function TaskModal({
           due = `${dueDate}T${dueTime}:00`;
           showWithoutTime = false;
         } else {
-          due = `${dueDate}T00:00:00`;
+          due = dueDate;
           showWithoutTime = true;
         }
       }
diff --git a/hooks/use-calendar-alerts.ts b/hooks/use-calendar-alerts.ts
index 6f9feba8..733fe91d 100644
--- a/hooks/use-calendar-alerts.ts
+++ b/hooks/use-calendar-alerts.ts
@@ -2,6 +2,7 @@
 
 import { useEffect, useRef, useCallback } from 'react';
 import { useTranslations, useLocale } from 'next-intl';
+import { parseISO } from 'date-fns';
 import { useAuthStore } from '@/stores/auth-store';
 import { useCalendarStore } from '@/stores/calendar-store';
 import { useSettingsStore } from '@/stores/settings-store';
@@ -50,7 +51,7 @@ export function useCalendarAlerts() {
           playNotificationSound(notificationSoundChoice);
         }
 
-        const diffMs = new Date(alert.event.utcStart || alert.event.start).getTime() - now;
+        const diffMs = (alert.event.utcStart ? new Date(alert.event.utcStart).getTime() : parseISO(alert.event.start).getTime()) - now;
         const diffMin = Math.round(diffMs / 60000);
 
         const timeLabel = diffMin <= 0
diff --git a/lib/calendar-alerts.ts b/lib/calendar-alerts.ts
index ac29bacb..69de556f 100644
--- a/lib/calendar-alerts.ts
+++ b/lib/calendar-alerts.ts
@@ -1,3 +1,4 @@
+import { parseISO } from 'date-fns';
 import type {
   CalendarEvent,
   CalendarEventAlert,
@@ -53,7 +54,7 @@ export function computeFireTime(
       baseTime = new Date(event.utcEnd).getTime();
     } else {
       // Compute end from start + duration
-      const startMs = new Date(event.start).getTime();
+      const startMs = parseISO(event.start).getTime();
       if (Number.isNaN(startMs)) return null;
       const durationMin = parseDuration(event.duration);
       baseTime = startMs + durationMin * 60000;
@@ -61,7 +62,7 @@ export function computeFireTime(
   } else {
     baseTime = event.utcStart
       ? new Date(event.utcStart).getTime()
-      : new Date(event.start).getTime();
+      : parseISO(event.start).getTime();
   }
 
   if (Number.isNaN(baseTime)) return null;
diff --git a/lib/calendar-invitation.ts b/lib/calendar-invitation.ts
index 5925aadf..b1dbad26 100644
--- a/lib/calendar-invitation.ts
+++ b/lib/calendar-invitation.ts
@@ -1,3 +1,4 @@
+import { parseISO } from 'date-fns';
 import type { Email, Attachment, CalendarEvent, CalendarParticipant, EmailBodyPart } from '@/lib/jmap/types';
 
 export type InvitationMethod =
@@ -534,7 +535,7 @@ function addDurationToDate(start: string, duration: string, _timeZone?: string |
   const minutes = parseInt(match[4] || '0');
   const seconds = parseInt(match[5] || '0');
 
-  const date = new Date(start);
+  const date = parseISO(start);
   if (isNaN(date.getTime())) return null;
 
   const isUTC = start.endsWith('Z') || start.includes('+');
diff --git a/lib/calendar-utils.ts b/lib/calendar-utils.ts
index 17f63209..7a8e98f3 100644
--- a/lib/calendar-utils.ts
+++ b/lib/calendar-utils.ts
@@ -12,14 +12,14 @@ export interface CalendarWeekSegment {
 }
 
 export function getEventEndDate(event: CalendarEvent): Date {
-  const start = new Date(event.start);
+  const start = parseISO(event.start);
   if (!event.duration) return start;
   return new Date(start.getTime() + parseDuration(event.duration) * 60000);
 }
 
 export function getEventDisplayEndDate(event: CalendarEvent): Date {
   const end = getEventEndDate(event);
-  if (!event.showWithoutTime || end.getTime() <= new Date(event.start).getTime()) {
+  if (!event.showWithoutTime || end.getTime() <= parseISO(event.start).getTime()) {
     return end;
   }
   return subMilliseconds(end, 1);
@@ -27,7 +27,7 @@ export function getEventDisplayEndDate(event: CalendarEvent): Date {
 
 export function getEventDayBounds(event: CalendarEvent): { startDay: Date; endDay: Date } {
   return {
-    startDay: startOfDay(new Date(event.start)),
+    startDay: startOfDay(parseISO(event.start)),
     endDay: startOfDay(getEventDisplayEndDate(event)),
   };
 }
@@ -77,7 +77,7 @@ export function buildWeekSegments(events: CalendarEvent[], weekDays: Date[]): Ca
     if (left.event.showWithoutTime !== right.event.showWithoutTime) {
       return left.event.showWithoutTime ? -1 : 1;
     }
-    const timeDiff = new Date(left.event.start).getTime() - new Date(right.event.start).getTime();
+    const timeDiff = parseISO(left.event.start).getTime() - parseISO(right.event.start).getTime();
     if (timeDiff !== 0) return timeDiff;
     return (left.event.title || "").localeCompare(right.event.title || "");
   });
@@ -100,7 +100,7 @@ export function layoutOverlappingEvents(
   events: CalendarEvent[],
 ): { event: CalendarEvent; column: number; totalColumns: number }[] {
   const sorted = [...events].sort((a, b) => {
-    const diff = new Date(a.start).getTime() - new Date(b.start).getTime();
+    const diff = parseISO(a.start).getTime() - parseISO(b.start).getTime();
     if (diff !== 0) return diff;
     return parseDuration(b.duration) - parseDuration(a.duration);
   });