feat: set a calendar's default reminders and start new events with them

src/api/__tests__/live-stalwart-calendar.test.ts

@@ -41,6 +41,8 @@ import { editorTimesChanged, editorTimesFromEvent, eventTimeFieldsToSave } from
 import { participantFieldsToSave, participantLimitOf, seedAttendees, type Attendee } from '../../lib/calendar-participants';
 import { expandRecurringEvents } from '../../lib/recurrence-expansion';
 import { getEventStartDate } from '../../lib/calendar-utils';
+import { alertsToReminders, calendarDefaultAlerts, defaultAlertIdPrefix, remindersToDefaultAlerts } from '../../lib/calendar-alerts';
+import { getEffectiveAlerts } from '../../lib/calendar-alert-scheduler';
 
 const SERVER = process.env.BULWARK_LIVE_JMAP;
 const USER = process.env.BULWARK_LIVE_USER ?? 'usera@example.org';
@@ -698,6 +700,60 @@ live('live Stalwart calendar', () => {
     }, 60_000);
   });
 
+  describe('a calendar\'s default reminders (#1209)', () => {
+    let defaultsCalendarId = '';
+    let otherCalendarId = '';
+
+    afterAll(async () => {
+      const ids = [defaultsCalendarId, otherCalendarId].filter(Boolean);
+      if (ids.length > 0) {
+        await raw(client, [['Calendar/set', { accountId: client.accountId, destroy: ids, onDestroyRemoveEvents: true }, '0']]).catch(() => undefined);
+      }
+    }, 30_000);
+
+    it('are set and cleared in the calendar and given to a new event that uses them', async () => {
+      await connectSingleton();
+      const [[, made]] = await raw(client, [['Calendar/set', {
+        accountId: client.accountId, create: { c: { name: `${RUN} defaults`, color: '#3b82f6' } },
+      }, '0']]);
+      defaultsCalendarId = made.created?.c?.id ?? '';
+      expect(defaultsCalendarId, JSON.stringify(made.notCreated)).toBeTruthy();
+      await useCalendarStore.getState().fetchCalendars();
+
+      // As the calendar editor saves them.
+      // Another calendar of the account with defaults under the plain ids
+      // the editor wrote before: 1.0 wants them unique in the account.
+      const plain = await raw(client, [['Calendar/set', {
+        accountId: client.accountId,
+        create: { o: { name: `${RUN} other defaults`, defaultAlertsWithTime: remindersToDefaultAlerts([{ minutesBefore: 5 }]) } },
+      }, '0']]);
+      otherCalendarId = plain[0][1].created?.o?.id ?? '';
+      const target = { id: defaultsCalendarId };
+      await useCalendarStore.getState().updateCalendar(defaultsCalendarId, {
+        defaultAlertsWithTime: remindersToDefaultAlerts([{ minutesBefore: 15 }], {}, defaultAlertIdPrefix(target, false)),
+        defaultAlertsWithoutTime: remindersToDefaultAlerts([{ minutesBefore: 60 * 24 }], {}, defaultAlertIdPrefix(target, true)),
+      });
+      await useCalendarStore.getState().fetchCalendars();
+      const cal = useCalendarStore.getState().calendars.find((c) => c.id === defaultsCalendarId)!;
+      expect(alertsToReminders(calendarDefaultAlerts(cal, false))).toEqual([{ minutesBefore: 15 }]);
+      expect(alertsToReminders(calendarDefaultAlerts(cal, true))).toEqual([{ minutesBefore: 60 * 24 }]);
+
+      // As the event editor creates an event on the defaults.
+      const created = await useCalendarStore.getState().createEvent({
+        '@type': 'Event', title: `${RUN} on defaults`, start: '2027-12-03T09:00:00', duration: 'PT1H', timeZone: 'Europe/Berlin', useDefaultAlerts: true,
+      } as Partial<CalendarEvent>, defaultsCalendarId);
+      const [stored] = await getEvents([created.id]);
+      expect(stored.useDefaultAlerts).toBe(true);
+      const effective = getEffectiveAlerts(stored, useCalendarStore.getState().calendars) ?? {};
+      expect(alertsToReminders(effective)).toEqual([{ minutesBefore: 15 }]);
+
+      // Emptied.
+      await useCalendarStore.getState().updateCalendar(defaultsCalendarId, { defaultAlertsWithTime: remindersToDefaultAlerts([]) });
+      const [[, after]] = await raw(client, [['Calendar/get', { accountId: client.accountId, ids: [defaultsCalendarId], properties: ['defaultAlertsWithTime'] }, '0']]);
+      expect(Object.keys(after.list[0].defaultAlertsWithTime ?? {})).toEqual([]);
+    }, 60_000);
+  });
+
   describe('a zero-length event (#837)', () => {
     it('ends at its start, stored without a duration or imported from an .ics', async () => {
       await connectSingleton();

src/api/calendar.ts

@@ -1,6 +1,6 @@
 import { jmapClient } from './jmap-client';
 import { CAPABILITIES } from './types';
-import type { Calendar, CalendarEvent, CalendarRights } from './types';
+import type { Alert, Calendar, CalendarEvent, CalendarRights } from './types';
 import { JMAPSetError, assertSetResult, batched, jmapErrorType, requireMethodResult } from './jmap-result';
 import { formatWallClock, getEffectiveTimeZone, getTimeZoneOffsetMs } from '../lib/calendar-timezone';
 import { SCAN_PROPERTIES, isTaskLikeObject, type ScannedCalendarObject } from '../lib/calendar-component-detection';
@@ -1295,6 +1295,9 @@ export interface CalendarUpdates {
   description?: string | null;
   sortOrder?: number;
   isSubscribed?: boolean;
+  // `{}` clears them: Stalwart refuses null for these maps.
+  defaultAlertsWithTime?: Record<string, Alert>;
+  defaultAlertsWithoutTime?: Record<string, Alert>;
 }
 
 // Rename / recolour / describe a calendar (Calendar/set update).

src/components/calendar/CalendarEditSheet.tsx

@@ -10,11 +10,22 @@ import {
   TouchableWithoutFeedback,
   View,
 } from 'react-native';
-import { Check } from '../icons';
-import type { Calendar } from '../../api/types';
+import { Check, Plus, X } from '../icons';
+import type { Alert, Calendar } from '../../api/types';
+import {
+  REMINDER_PRESETS,
+  alertsToReminders,
+  calendarDefaultAlerts,
+  defaultAlertIdPrefix,
+  formatReminder,
+  preservedAlerts,
+  remindersToDefaultAlerts,
+  sameReminders,
+  type Reminder,
+} from '../../lib/calendar-alerts';
 import { radius, spacing, typography, type ThemePalette } from '../../theme/tokens';
 import { useColors } from '../../theme/colors';
-import { useLocaleStore } from '../../stores/locale-store';
+import { useLocaleStore, type TranslateFn } from '../../stores/locale-store';
 import { CALENDAR_COLOR_PALETTE, calendarColorName, getCalendarColor } from '../../lib/calendar-utils';
 import Button from '../Button';
 import { MODAL_ORIENTATIONS } from '../../lib/modal-orientations';
@@ -24,6 +35,79 @@ export interface CalendarEditValues {
   name: string;
   color: string;
   description: string;
+  /** Set only when the user changed them. */
+  defaultAlertsWithTime?: Record<string, Alert>;
+  defaultAlertsWithoutTime?: Record<string, Alert>;
+}
+
+/** One list of a calendar's default reminders: the times, a remove button each, presets to add. */
+function DefaultRemindersEditor({
+  label,
+  reminders,
+  keptCount,
+  onChange,
+  styles,
+  c,
+  t,
+}: {
+  label: string;
+  reminders: Reminder[];
+  keptCount: number;
+  onChange: (next: Reminder[]) => void;
+  styles: ReturnType<typeof makeStyles>;
+  c: ThemePalette;
+  t: TranslateFn;
+}) {
+  const [picking, setPicking] = React.useState(false);
+  const available = REMINDER_PRESETS.filter((p) => !reminders.some((r) => r.minutesBefore === p));
+  return (
+    <View style={styles.reminders}>
+      <Text style={styles.label}>{label}</Text>
+      {reminders.length === 0 && keptCount === 0 && (
+        <Text style={styles.muted}>{t('calendar.alerts.none', 'No reminders')}</Text>
+      )}
+      {reminders.map((r) => (
+        <View key={r.minutesBefore} style={styles.reminderRow}>
+          <Text style={styles.reminderText}>{formatReminder(r.minutesBefore, t)}</Text>
+          <Pressable
+            onPress={() => onChange(reminders.filter((x) => x.minutesBefore !== r.minutesBefore))}
+            hitSlop={8}
+            accessibilityRole="button"
+            accessibilityLabel={t('calendar.alerts.remove', 'Remove reminder')}
+          >
+            <X size={16} color={c.textMuted} />
+          </Pressable>
+        </View>
+      ))}
+      {keptCount > 0 && (
+        <Text style={styles.muted}>
+          {t('calendar.alerts.kept', "{count, plural, one {# more reminder that can't be edited here is kept} other {# more reminders that can't be edited here are kept}}", { count: keptCount })}
+        </Text>
+      )}
+      {available.length > 0 && (
+        <Pressable style={styles.addRow} onPress={() => setPicking((v) => !v)} accessibilityRole="button">
+          <Plus size={16} color={c.primary} />
+          <Text style={styles.addRowText}>{t('calendar.alerts.add', 'Add reminder')}</Text>
+        </Pressable>
+      )}
+      {picking && (
+        <View style={styles.presets}>
+          {available.map((preset) => (
+            <Pressable
+              key={preset}
+              style={styles.preset}
+              onPress={() => {
+                onChange([...reminders, { minutesBefore: preset }]);
+                setPicking(false);
+              }}
+            >
+              <Text style={styles.presetText}>{formatReminder(preset, t)}</Text>
+            </Pressable>
+          ))}
+        </View>
+      )}
+    </View>
+  );
 }
 
 interface CalendarEditSheetProps {
@@ -47,15 +131,26 @@ export function CalendarEditSheet({ visible, calendar, onSave, onClose }: Calend
   const [description, setDescription] = React.useState('');
   const [saving, setSaving] = React.useState(false);
   const [error, setError] = React.useState<string | null>(null);
+  // The calendar's default reminders (#1209): the owner's to set, for an
+  // existing calendar. What the lists can't show is written back unchanged.
+  const editsDefaults = !!calendar && !calendar.isShared;
+  const initialTimed = React.useMemo(() => alertsToReminders(calendarDefaultAlerts(calendar, false)), [calendar]);
+  const initialAllDay = React.useMemo(() => alertsToReminders(calendarDefaultAlerts(calendar, true)), [calendar]);
+  const keptTimed = React.useMemo(() => preservedAlerts(calendarDefaultAlerts(calendar, false)), [calendar]);
+  const keptAllDay = React.useMemo(() => preservedAlerts(calendarDefaultAlerts(calendar, true)), [calendar]);
+  const [timedReminders, setTimedReminders] = React.useState<Reminder[]>([]);
+  const [allDayReminders, setAllDayReminders] = React.useState<Reminder[]>([]);
 
   React.useEffect(() => {
     if (!visible) return;
     setName(calendar?.name ?? '');
     setColor(calendar ? getCalendarColor(calendar) : CALENDAR_COLOR_PALETTE[0]);
     setDescription(calendar?.description ?? '');
+    setTimedReminders(initialTimed);
+    setAllDayReminders(initialAllDay);
     setError(null);
     setSaving(false);
-  }, [visible, calendar]);
+  }, [visible, calendar, initialTimed, initialAllDay]);
 
   const handleSave = async () => {
     const trimmed = name.trim();
@@ -63,7 +158,15 @@ export function CalendarEditSheet({ visible, calendar, onSave, onClose }: Calend
     setSaving(true);
     setError(null);
     try {
-      await onSave({ name: trimmed, color, description: description.trim() });
+      const values: CalendarEditValues = { name: trimmed, color, description: description.trim() };
+      // Ids unique in the account: Stalwart 1.0 refuses one another calendar uses.
+      if (calendar && editsDefaults && !sameReminders(timedReminders, initialTimed)) {
+        values.defaultAlertsWithTime = remindersToDefaultAlerts(timedReminders, keptTimed, defaultAlertIdPrefix(calendar, false));
+      }
+      if (calendar && editsDefaults && !sameReminders(allDayReminders, initialAllDay)) {
+        values.defaultAlertsWithoutTime = remindersToDefaultAlerts(allDayReminders, keptAllDay, defaultAlertIdPrefix(calendar, true));
+      }
+      await onSave(values);
       onClose();
     } catch (e) {
       setError(e instanceof Error ? e.message : t('calendar.notifications.event_error', 'Something went wrong'));
@@ -120,6 +223,35 @@ export function CalendarEditSheet({ visible, calendar, onSave, onClose }: Calend
                   style={[styles.input, styles.multiline]}
                   multiline
                 />
+                {editsDefaults && (
+                  <>
+                    <Text style={styles.sectionTitle}>{t('calendar.management.default_alerts', 'Default reminders')}</Text>
+                    <Text style={styles.muted}>
+                      {t(
+                        'calendar.management.default_alerts_hint',
+                        "New events in this calendar get these reminders unless you change them in the event. Changing them here doesn't change existing events. Email reminders are sent by the mail server.",
+                      )}
+                    </Text>
+                    <DefaultRemindersEditor
+                      label={t('calendar.management.default_alerts_timed', 'Events with a time')}
+                      reminders={timedReminders}
+                      keptCount={Object.keys(keptTimed).length}
+                      onChange={setTimedReminders}
+                      styles={styles}
+                      c={c}
+                      t={t}
+                    />
+                    <DefaultRemindersEditor
+                      label={t('calendar.management.default_alerts_all_day', 'All-day events')}
+                      reminders={allDayReminders}
+                      keptCount={Object.keys(keptAllDay).length}
+                      onChange={setAllDayReminders}
+                      styles={styles}
+                      c={c}
+                      t={t}
+                    />
+                  </>
+                )}
                 {error ? <Text style={styles.error}>{error}</Text> : null}
               </ScrollView>
               <View style={styles.footer}>
@@ -185,5 +317,32 @@ function makeStyles(c: ThemePalette) {
     swatchActive: { borderWidth: 2, borderColor: c.text },
     error: { ...typography.caption, color: c.error },
     footer: { flexDirection: 'row', justifyContent: 'flex-end', gap: spacing.md },
+    sectionTitle: { ...typography.bodyMedium, color: c.text, marginTop: spacing.md },
+    muted: { ...typography.caption, color: c.textMuted },
+    reminders: { gap: spacing.xs },
+    reminderRow: {
+      flexDirection: 'row',
+      alignItems: 'center',
+      justifyContent: 'space-between',
+      minHeight: 36,
+      paddingHorizontal: spacing.md,
+      borderRadius: radius.sm,
+      borderWidth: 1,
+      borderColor: c.border,
+      backgroundColor: c.surface,
+    },
+    reminderText: { ...typography.body, color: c.text },
+    addRow: { flexDirection: 'row', alignItems: 'center', gap: spacing.xs, paddingVertical: spacing.xs },
+    addRowText: { ...typography.bodyMedium, color: c.primary },
+    presets: { flexDirection: 'row', flexWrap: 'wrap', gap: spacing.xs },
+    preset: {
+      paddingHorizontal: spacing.md,
+      paddingVertical: spacing.xs,
+      borderRadius: radius.full,
+      borderWidth: 1,
+      borderColor: c.border,
+      backgroundColor: c.surface,
+    },
+    presetText: { ...typography.caption, color: c.text },
   });
 }

src/components/calendar/EventModal.tsx

@@ -50,8 +50,10 @@ import { useCalendarLocale } from '../../lib/calendar-locale';
 import { formatCalendarDate } from '../../lib/date-format';
 import {
   alertsToReminders,
+  calendarDefaultAlerts,
   reminderFieldsToSave,
   preservedAlerts,
+  withOwnAlertIds,
   formatReminder,
   REMINDER_PRESETS,
   REMINDER_UNIT_MINUTES,
@@ -228,6 +230,10 @@ export function EventModal({
   const [customRule, setCustomRule] = React.useState<RecurrenceRule | null>(null);
   const [recurrenceEditorOpen, setRecurrenceEditorOpen] = React.useState(false);
   const [reminders, setReminders] = React.useState<Reminder[]>([]);
+  // A new event starts on its calendar's default reminders (#1209) until
+  // the user customizes them.
+  const [useDefaults, setUseDefaults] = React.useState(false);
+  const onDefaults = !event && useDefaults;
   // Alerts the picker can't show (absolute / end-relative / non-display)
   // ride along untouched so a save never drops them.
   const [preserved, setPreserved] = React.useState<Record<string, Alert>>({});
@@ -268,6 +274,7 @@ export function EventModal({
       setCustomRule(detected === 'custom' ? event.recurrenceRules?.[0] ?? null : null);
       setRecurrenceEditorOpen(false);
       setReminders(alertsToReminders(event.alerts));
+      setUseDefaults(false);
       setPreserved(preservedAlerts(event.alerts));
       setSendInvitations(true);
       setCustomReminderOpen(false);
@@ -286,6 +293,7 @@ export function EventModal({
       setCustomRule(null);
       setRecurrenceEditorOpen(false);
       setReminders([]);
+      setUseDefaults(true);
       setPreserved({});
       setSendInvitations(true);
       setCustomReminderOpen(false);
@@ -369,7 +377,11 @@ export function EventModal({
         recurrenceRules: clearedOr(recurrenceRules, 'recurrenceRules'),
         // Untouched reminders are not sent: an event on its calendar's
         // default alerts keeps them.
-        ...reminderFieldsToSave(event, reminders, preserved, defaultAlertIds),
+        // A new event on its calendar's defaults: the server copies them
+        // into it (sending them as alerts too would double them).
+        ...(onDefaults
+          ? { useDefaultAlerts: true }
+          : reminderFieldsToSave(event, reminders, preserved, defaultAlertIds)),
         locations: clearedOr(buildLocations(location), 'locations'),
         virtualLocations: clearedOr(buildVirtualLocations(videoUrl), 'virtualLocations'),
       };
@@ -455,6 +467,18 @@ export function EventModal({
   const availablePresets = REMINDER_PRESETS.filter(
     (p) => !reminders.some((r) => r.minutesBefore === p),
   );
+  // The selected calendar's defaults, for an event with a time or all day.
+  const calendarDefaults = calendarDefaultAlerts(selectedCalendar, allDay);
+  const defaultReminders = alertsToReminders(calendarDefaults);
+  // Start the event's own reminders from the defaults, to change them.
+  const customizeReminders = () => {
+    setReminders(defaultReminders);
+    // Under ids of their own: Stalwart 1.0 drops an event's alert under a
+    // default alert's id.
+    const kept = preservedAlerts(calendarDefaults);
+    setPreserved(withOwnAlertIds(Object.values(kept), Object.keys(calendarDefaults)));
+    setUseDefaults(false);
+  };
 
   return (
     <SafeAreaModal supportedOrientations={MODAL_ORIENTATIONS}
@@ -654,7 +678,36 @@ export function EventModal({
           </Section>
 
           <Section icon={<Bell size={16} color={c.textMuted} />} label={t('calendar.alerts.title', 'Reminders')}>
-            {reminders.length > 0 && (
+            {onDefaults && (
+              <View style={{ gap: spacing.xs }}>
+                <Text style={styles.defaultsCaption}>{t('calendar.alerts.calendar_defaults', 'Calendar defaults')}</Text>
+                {defaultReminders.length === 0 && (
+                  <Text style={styles.defaultsCaption}>{t('calendar.alerts.none', 'No reminders')}</Text>
+                )}
+                {defaultReminders.map((r) => (
+                  <View key={r.minutesBefore} style={styles.reminderRow}>
+                    <Text style={styles.fieldText}>{formatReminder(r.minutesBefore, t)}</Text>
+                  </View>
+                ))}
+                <Pressable style={styles.addRow} onPress={customizeReminders} accessibilityRole="button">
+                  <Text style={styles.addRowText}>{t('calendar.alerts.customize', 'Customize')}</Text>
+                </Pressable>
+              </View>
+            )}
+            {!onDefaults && !event && (
+              <Pressable
+                style={styles.addRow}
+                onPress={() => {
+                  setUseDefaults(true);
+                  setReminderPickerOpen(false);
+                  setCustomReminderOpen(false);
+                }}
+                accessibilityRole="button"
+              >
+                <Text style={styles.addRowText}>{t('calendar.alerts.use_defaults', 'Use calendar defaults')}</Text>
+              </Pressable>
+            )}
+            {!onDefaults && reminders.length > 0 && (
               <View style={{ gap: spacing.xs }}>
                 {reminders.map((r) => (
                   <View key={r.minutesBefore} style={styles.reminderRow}>
@@ -672,7 +725,7 @@ export function EventModal({
                 ))}
               </View>
             )}
-            {availablePresets.length > 0 && (
+            {!onDefaults && availablePresets.length > 0 && (
               <Pressable
                 style={styles.addRow}
                 onPress={() => setReminderPickerOpen((v) => !v)}
@@ -1035,6 +1088,7 @@ function makeStyles(c: ThemePalette) {
     paddingVertical: spacing.sm,
   },
   addRowText: { ...typography.body, color: c.primary },
+  defaultsCaption: { ...typography.caption, color: c.textMuted },
 
   popover: {
     borderWidth: 1,

src/lib/__tests__/calendar-alerts.test.ts

@@ -8,6 +8,10 @@ import {
   preservedAlerts,
   reminderFieldsToSave,
   withOwnAlertIds,
+  calendarDefaultAlerts,
+  defaultAlertIdPrefix,
+  remindersToDefaultAlerts,
+  sameReminders,
 } from '../calendar-alerts';
 import { translate, type MessageParams } from '../../i18n';
 import type { Alert } from '../../api/types';
@@ -164,3 +168,41 @@ describe('withOwnAlertIds', () => {
     expect(Object.keys(withOwnAlertIds([alert, alert], ['reminder-1', 'reminder-3']))).toEqual(['reminder-2', 'reminder-4']);
   });
 });
+
+describe('a calendar\'s default reminders (#1209)', () => {
+  const email = { '@type': 'Alert', action: 'email', trigger: { '@type': 'OffsetTrigger', offset: '-PT1H' } } as Alert;
+
+  it('picks the defaults for timed or all-day events', () => {
+    const cal = {
+      defaultAlertsWithTime: { a: { '@type': 'Alert', trigger: { '@type': 'OffsetTrigger', offset: '-PT15M' } } as Alert },
+      defaultAlertsWithoutTime: null,
+    };
+    expect(alertsToReminders(calendarDefaultAlerts(cal, false))).toEqual([{ minutesBefore: 15 }]);
+    expect(calendarDefaultAlerts(cal, true)).toEqual({});
+    expect(calendarDefaultAlerts(undefined, false)).toEqual({});
+  });
+
+  it('writes the defaults under ids of their own and keeps what the list cannot show', () => {
+    const alerts = remindersToDefaultAlerts([{ minutesBefore: 10 }, { minutesBefore: 60 * 24 }], { 'default-1': email });
+    expect(alerts['default-1']).toBe(email);
+    expect(Object.keys(alerts).sort()).toEqual(['default-1', 'default-2', 'default-3']);
+    expect(alertsToReminders(alerts)).toEqual([{ minutesBefore: 60 * 24 }, { minutesBefore: 60 }, { minutesBefore: 10 }]);
+    // Emptied: a map Stalwart takes, not null.
+    expect(remindersToDefaultAlerts([])).toEqual({});
+  });
+
+  it('gives each calendar and list ids no other calendar of the account uses (Stalwart 1.0)', () => {
+    const b = remindersToDefaultAlerts([{ minutesBefore: 15 }], {}, defaultAlertIdPrefix({ id: 'b' }, false));
+    const c = remindersToDefaultAlerts([{ minutesBefore: 15 }], {}, defaultAlertIdPrefix({ id: 'acc-2:c', originalId: 'c' }, false));
+    const bAllDay = remindersToDefaultAlerts([{ minutesBefore: 15 }], {}, defaultAlertIdPrefix({ id: 'b' }, true));
+    const ids = [...Object.keys(b), ...Object.keys(c), ...Object.keys(bAllDay)];
+    expect(new Set(ids).size).toBe(3);
+    expect(Object.keys(b)).toEqual(['cal-b-timed-1']);
+  });
+
+  it('tells a changed list from the same times in another order', () => {
+    expect(sameReminders([{ minutesBefore: 5 }, { minutesBefore: 15 }], [{ minutesBefore: 15 }, { minutesBefore: 5 }])).toBe(true);
+    expect(sameReminders([{ minutesBefore: 5 }], [{ minutesBefore: 10 }])).toBe(false);
+    expect(sameReminders([], [{ minutesBefore: 10 }])).toBe(false);
+  });
+});

src/lib/calendar-alerts.ts

@@ -1,4 +1,4 @@
-import type { Alert, CalendarEvent } from '../api/types';
+import type { Alert, Calendar, CalendarEvent } from '../api/types';
 import { formatMessage, type MessageParams } from '../i18n/format';
 
 // A reminder, expressed as "minutes before the event start". 0 means "at the
@@ -130,6 +130,57 @@ function reminderMinutes(reminders: Reminder[]): number[] {
   return [...new Set(reminders.map((r) => r.minutesBefore))].sort((a, b) => b - a);
 }
 
+/** Whether two reminder lists remind at the same times. */
+export function sameReminders(a: Reminder[], b: Reminder[]): boolean {
+  const x = reminderMinutes(a);
+  const y = reminderMinutes(b);
+  return x.length === y.length && x.every((minutes, i) => minutes === y[i]);
+}
+
+// ─── A calendar's default alerts (#1209) ─────────────────
+
+/** The alerts `calendar` gives an event with a time, or an all-day one, that uses its defaults. */
+export function calendarDefaultAlerts(
+  calendar: Pick<Calendar, 'defaultAlertsWithTime' | 'defaultAlertsWithoutTime'> | null | undefined,
+  allDay: boolean,
+): Record<string, Alert> {
+  return (allDay ? calendar?.defaultAlertsWithoutTime : calendar?.defaultAlertsWithTime) ?? {};
+}
+
+/**
+ * A calendar's default alerts as the editor's reminders set them, with the
+ * ones it can't show (`preserved`) kept as they were. Under ids of their own,
+ * `${idPrefix}-1` and on: Stalwart 1.0 refuses default alert ids that another
+ * calendar of the account uses ("Default alert ids must be unique across all
+ * calendars in the account."), so the prefix names the calendar and the
+ * list; it also shows them in an event that uses them under the same ids,
+ * where an event's own reminder-N alerts must not meet them. An emptied list
+ * is `{}`: Stalwart refuses null for these maps.
+ */
+/** The id prefix of `calendar`'s default alerts for timed or all-day events, unique in its account. */
+export function defaultAlertIdPrefix(calendar: Pick<Calendar, 'id' | 'originalId'>, allDay: boolean): string {
+  return `cal-${calendar.originalId || calendar.id}-${allDay ? 'allday' : 'timed'}`;
+}
+
+export function remindersToDefaultAlerts(
+  reminders: Reminder[],
+  preserved: Record<string, Alert> = {},
+  idPrefix = 'default',
+): Record<string, Alert> {
+  const alerts: Record<string, Alert> = { ...preserved };
+  let n = 0;
+  for (const minutesBefore of reminderMinutes(reminders)) {
+    let key = `${idPrefix}-${++n}`;
+    while (key in alerts) key = `${idPrefix}-${++n}`;
+    alerts[key] = {
+      '@type': 'Alert',
+      trigger: { '@type': 'OffsetTrigger', offset: minutesBeforeToOffset(minutesBefore), relativeTo: 'start' },
+      action: 'display',
+    };
+  }
+  return alerts;
+}
+
 /**
  * What an editor save writes for the reminders: nothing while they are the
  * ones the event had, so an event that uses its calendar's default alerts

src/screens/CalendarScreen.tsx

@@ -93,7 +93,7 @@ import { buildReplyTo } from '../lib/calendar-invitation';
 import { isRecurringSeriesMember } from '../lib/recurrence-overrides';
 import { deleteOccurrence, saveOccurrenceEdit, type SeriesEdit } from '../lib/recurrence-scope-edit';
 import { participantLimitMessage } from '../lib/calendar-participants';
-import { maxParticipantsPerEvent } from '../api/calendar';
+import { maxParticipantsPerEvent, type CalendarUpdates } from '../api/calendar';
 import { useContactNameResolver } from '../lib/contact-name-resolver';
 import { generateBirthdayEvents, createBirthdayCalendar, BIRTHDAY_CALENDAR_ID } from '../lib/birthday-calendar';
 import { moveCalendar } from '../lib/calendar-sort';
@@ -880,7 +880,10 @@ export default function CalendarScreen() {
       } else {
         // Only what the user changed: an untouched colour another client
         // wrote may be one Stalwart 1.0 refuses.
-        const changes = changedCalendarFields(calendarEditTarget.calendar, values);
+        const changes: CalendarUpdates = changedCalendarFields(calendarEditTarget.calendar, values);
+        // The default reminders come only when the user changed them.
+        if (values.defaultAlertsWithTime) changes.defaultAlertsWithTime = values.defaultAlertsWithTime;
+        if (values.defaultAlertsWithoutTime) changes.defaultAlertsWithoutTime = values.defaultAlertsWithoutTime;
         if (Object.keys(changes).length > 0) await updateCalendar(calendarEditTarget.calendar.id, changes);
       }
     },