diff --git a/components/calendar/event-card.tsx b/components/calendar/event-card.tsx
index 902a6e5c..f797f0ea 100644
--- a/components/calendar/event-card.tsx
+++ b/components/calendar/event-card.tsx
@@ -78,7 +78,11 @@ export function EventCard({ event, calendar, variant, onClick, onMouseEnter, onM
   const calendarName = calendar?.name || "";
   const durationMinutes = parseDuration(event.duration);
   const endTime = getEventEndDate(event);
-  const timeString = `${format(startDate, timeFmt)} – ${format(endTime, timeFmt)}`;
+  const safeFormat = (d: Date, fmt: string) => {
+    if (isNaN(d.getTime())) return "--:--";
+    try { return format(d, fmt); } catch { return "--:--"; }
+  };
+  const timeString = `${safeFormat(startDate, timeFmt)} – ${safeFormat(endTime, timeFmt)}`;
   const ariaLabel = `${event.title || t("events.no_title")}, ${timeString}${calendarName ? `, ${calendarName}` : ""}`;
 
   const handleDragStart = useCallback((e: DragEvent) => {
diff --git a/lib/calendar-utils.ts b/lib/calendar-utils.ts
index 64179fb3..a6bcac0a 100644
--- a/lib/calendar-utils.ts
+++ b/lib/calendar-utils.ts
@@ -24,8 +24,14 @@ export interface TimedEventLayout {
 export function getEventStartDate(
   event: Pick<CalendarEvent, 'start' | 'utcStart' | 'showWithoutTime'>,
 ): Date {
-  const source = !event.showWithoutTime && event.utcStart ? event.utcStart : event.start;
-  return parseISO(source);
+  // Prefer utcStart for timed events but fall back to start if utcStart is
+  // missing or unparseable - a malformed utcStart used to surface as an
+  // Invalid Date that crashed downstream format() calls (#316).
+  if (!event.showWithoutTime && event.utcStart) {
+    const utc = parseISO(event.utcStart);
+    if (!isNaN(utc.getTime())) return utc;
+  }
+  return parseISO(event.start);
 }
 
 export function packWeekSegments(rawSegments: CalendarWeekSegment[]): CalendarWeekSegment[] {
@@ -56,7 +62,8 @@ export function packWeekSegments(rawSegments: CalendarWeekSegment[]): CalendarWe
 
 export function getEventEndDate(event: CalendarEvent): Date {
   if (!event.showWithoutTime && event.utcEnd) {
-    return parseISO(event.utcEnd);
+    const utc = parseISO(event.utcEnd);
+    if (!isNaN(utc.getTime())) return utc;
   }
 
   const start = getEventStartDate(event);
diff --git a/stores/calendar-store.ts b/stores/calendar-store.ts
index 1fb94a40..3d21a554 100644
--- a/stores/calendar-store.ts
+++ b/stores/calendar-store.ts
@@ -7,6 +7,7 @@ import { normalizeAllDayDuration } from '@/lib/calendar-utils';
 import { parseDuration } from '@/components/calendar/event-card';
 import { sanitizeOutgoingCalendarEventData } from '@/lib/calendar-event-normalization';
 import { expandRecurringEvents } from '@/lib/recurrence-expansion';
+import { parseISO } from 'date-fns';
 import { generateUUID } from '@/lib/utils';
 import { apiFetch } from '@/lib/browser-navigation';
 import { BIRTHDAY_CALENDAR_ID } from '@/lib/birthday-calendar';
@@ -302,8 +303,12 @@ export const useCalendarStore = create<CalendarStore>()(
             after: start,
             before: end,
           });
-          // Filter out malformed events missing required 'start' field
-          const validEvents = rawEvents.filter(e => typeof e.start === 'string' && e.start);
+          // Filter out malformed events missing required 'start' field, or
+          // whose start string fails to parse (would otherwise crash format()
+          // calls in the rendering path - #316).
+          const validEvents = rawEvents.filter(e =>
+            typeof e.start === 'string' && e.start && !isNaN(parseISO(e.start).getTime())
+          );
           const droppedEvents = rawEvents.length - validEvents.length;
           // Expand recurring events client-side (Stalwart doesn't support
           // mutations on synthetic IDs from server-side expandRecurrences)
@@ -366,7 +371,9 @@ export const useCalendarStore = create<CalendarStore>()(
             accounts.map(async ({ client, localAccountId }) => {
               try {
                 const raw = await client.queryAllCalendarEvents({ after: start, before: end });
-                const valid = raw.filter(e => typeof e.start === 'string' && e.start);
+                const valid = raw.filter(e =>
+                  typeof e.start === 'string' && e.start && !isNaN(parseISO(e.start).getTime())
+                );
                 const expanded = expandRecurringEvents(valid, start, end);
                 return prefixEventsWithLocalAccount(
                   expanded,