feat(calendar): move the end with the start, keeping the event's length

Editing an event's start now shifts the end by the same amount, preserving the
current duration (end - start). Moving the start forward never leaves the end
before it, and moving it either way never silently changes the length. Guards
against an already-negative-duration state.

Extracted a pure `shiftedEnd(oldStart, oldEnd, newStart)` for the start
date/time onChange handlers. +7 unit tests. tsc + eslint clean.

components/calendar/event-modal.tsx

@@ -51,6 +51,19 @@ interface EventModalProps {
   isMobile?: boolean;
 }
 
+/**
+ * When an event's start is edited, the end moves with it to keep the SAME
+ * duration (oldEnd - oldStart), so the appointment's length is preserved.
+ * Returns the new end, or null for invalid or already-negative-duration input.
+ * Pure, for testing.
+ */
+export function shiftedEnd(oldStart: Date, oldEnd: Date, newStart: Date): Date | null {
+  if (isNaN(oldStart.getTime()) || isNaN(oldEnd.getTime()) || isNaN(newStart.getTime())) return null;
+  const durationMs = oldEnd.getTime() - oldStart.getTime();
+  if (durationMs < 0) return null;
+  return new Date(newStart.getTime() + durationMs);
+}
+
 function formatDateInput(d: Date): string {
   return format(d, "yyyy-MM-dd");
 }
@@ -257,6 +270,21 @@ export function EventModal({
   const [endDate, setEndDate] = useState(formatDateInput(getInitialEnd()));
   const [endTime, setEndTime] = useState(formatTimeInput(getInitialEnd()));
   const [allDay, setAllDay] = useState(event?.showWithoutTime || defaultAllDay || false);
+
+  // Editing the start shifts the end with it, preserving the event's current
+  // length (end - start) - so moving the start never leaves the end before it
+  // and never silently changes the duration.
+  const shiftEndKeepingDuration = (nextStartDate: string, nextStartTime: string) => {
+    const oldStart = new Date(`${startDate}T${allDay ? "00:00" : (startTime || "00:00")}:00`);
+    const oldEnd = new Date(`${endDate}T${allDay ? "00:00" : (endTime || "00:00")}:00`);
+    const newStart = new Date(`${nextStartDate}T${allDay ? "00:00" : (nextStartTime || "00:00")}:00`);
+    const nextEnd = shiftedEnd(oldStart, oldEnd, newStart);
+    if (!nextEnd) return;
+    setEndDate(formatDateInput(nextEnd));
+    if (!allDay) setEndTime(formatTimeInput(nextEnd));
+  };
+  const handleStartDateChange = (v: string) => { setStartDate(v); shiftEndKeepingDuration(v, startTime); };
+  const handleStartTimeChange = (v: string) => { setStartTime(v); shiftEndKeepingDuration(startDate, v); };
   const [calendarId, setCalendarId] = useState<string>(() => {
     if (event?.calendarIds) return getPrimaryCalendarId(event) || calendars[0]?.id || "";
     if (defaultCalendarId && calendars.some(c => c.id === defaultCalendarId)) return defaultCalendarId;
@@ -1097,7 +1125,7 @@ export function EventModal({
               <input
                 type="date"
                 value={startDate}
-                onChange={(e) => setStartDate(e.target.value)}
+                onChange={(e) => handleStartDateChange(e.target.value)}
                 className="w-full rounded-md border border-input bg-background px-3 py-2 text-sm focus:outline-none focus:ring-2 focus:ring-ring"
               />
             </div>
@@ -1107,7 +1135,7 @@ export function EventModal({
                 <input
                   type="time"
                   value={startTime}
-                  onChange={(e) => setStartTime(e.target.value)}
+                  onChange={(e) => handleStartTimeChange(e.target.value)}
                   className="w-full rounded-md border border-input bg-background px-3 py-2 text-sm focus:outline-none focus:ring-2 focus:ring-ring"
                 />
               </div>

lib/__tests__/event-end-follows-start.test.ts

@@ -0,0 +1,43 @@
+import { describe, it, expect } from 'vitest';
+import { shiftedEnd } from '@/components/calendar/event-modal';
+
+const d = (s: string) => new Date(s);
+
+describe('shiftedEnd (event editor: end follows start, keeping duration)', () => {
+  it('moves the end forward by the same duration when start moves forward', () => {
+    // 2h event 09:00-11:00, start -> 14:00  =>  end 16:00 (still 2h)
+    expect(shiftedEnd(d('2026-03-01T09:00:00'), d('2026-03-01T11:00:00'), d('2026-03-01T14:00:00')))
+      .toEqual(d('2026-03-01T16:00:00'));
+  });
+
+  it('moves the end backward by the same duration when start moves backward', () => {
+    // 1h event 10:00-11:00, start -> 08:00  =>  end 09:00 (still 1h)
+    expect(shiftedEnd(d('2026-03-01T10:00:00'), d('2026-03-01T11:00:00'), d('2026-03-01T08:00:00')))
+      .toEqual(d('2026-03-01T09:00:00'));
+  });
+
+  it('preserves a multi-hour duration exactly', () => {
+    // 90min event, start moved past the old end
+    expect(shiftedEnd(d('2026-03-01T09:00:00'), d('2026-03-01T10:30:00'), d('2026-03-01T12:00:00')))
+      .toEqual(d('2026-03-01T13:30:00'));
+  });
+
+  it('preserves a multi-day (all-day) duration', () => {
+    // 2-day span, start -> 03-10  =>  end 03-12
+    expect(shiftedEnd(d('2026-03-01T00:00:00'), d('2026-03-03T00:00:00'), d('2026-03-10T00:00:00')))
+      .toEqual(d('2026-03-12T00:00:00'));
+  });
+
+  it('keeps a zero-length event zero-length', () => {
+    expect(shiftedEnd(d('2026-03-01T10:00:00'), d('2026-03-01T10:00:00'), d('2026-03-01T15:00:00')))
+      .toEqual(d('2026-03-01T15:00:00'));
+  });
+
+  it('returns null for an already-negative duration (does not amplify a bad state)', () => {
+    expect(shiftedEnd(d('2026-03-01T11:00:00'), d('2026-03-01T10:00:00'), d('2026-03-01T14:00:00'))).toBeNull();
+  });
+
+  it('returns null for invalid dates', () => {
+    expect(shiftedEnd(new Date('nope'), d('2026-03-01T11:00:00'), d('2026-03-01T09:00:00'))).toBeNull();
+  });
+});