feat(reminders): custom date/time option in Remind Me

The Remind Me dialog only offered four fixed presets, so you couldn't set a
reminder for an arbitrary time. Add a "Custom time…" option that reveals date +
time pickers (validated >= 1 minute in the future) and sets the reminder at that
absolute timestamp.

Also extract the local date/time <input> helpers (toLocalDate, toLocalTime,
parseLocalDateTime, pickerInputStyle) into a shared, unit-tested
utils/datetimeInput.ts and reuse them in ScheduleMessageModal (deduped from an
inline copy) — identical output, now covered by tests.

Documents the previously-undocumented Message Reminders feature in
LOTUS_FEATURES.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

LOTUS_FEATURES.md

@@ -746,6 +746,12 @@ Redacted events display "This message has been deleted" along with the redaction
 - **Edit / reschedule**: the tray's edit button re-opens `ScheduleMessageModal` (seeded with the existing body + send-time) to change the text and/or time. Since MSC4140 has no in-place edit, this is implemented as schedule-new-then-cancel-old; the old copy is only removed once the server confirms cancellation, so a failed cancel leaves a visible, cancellable copy rather than losing the message. Edits go through the plain-text composer (rich content becomes `m.text`).
 - Utilities in `src/app/utils/scheduledMessages.ts`
 
+### Message Reminders
+
+- Message context menu → **Remind Me** sets a personal reminder to revisit a message; reminders are stored in `io.lotus.reminders` account data (sync across devices) via `useReminders`, and fire from `ClientNonUIFeatures`.
+- `RemindMeDialog.tsx` offers quick presets (in 20 min / 1 hour / 3 hours / tomorrow 9am) **plus a "Custom time…" option** that reveals date + time pickers for an arbitrary reminder time (validated to be ≥ 1 minute in the future).
+- The date/time input helpers (`toLocalDate`, `toLocalTime`, `parseLocalDateTime`, `pickerInputStyle`) are shared, pure, and unit-tested in `src/app/utils/datetimeInput.ts` (`datetimeInput.test.ts`) — also used by `ScheduleMessageModal` (deduped from a prior inline copy).
+
 ### File Upload Compression (opt-in)
 
 - Implemented in `UploadCardRenderer.tsx`

src/app/features/room/ScheduleMessageModal.tsx

@@ -21,6 +21,7 @@ import { useMatrixClient } from '../../hooks/useMatrixClient';
 import { stopPropagation } from '../../utils/keyboard';
 import { scheduleMessage } from '../../utils/scheduledMessages';
 import { useModalStyle } from '../../hooks/useModalStyle';
+import { toLocalDate, toLocalTime, parseLocalDateTime, pickerInputStyle } from '../../utils/datetimeInput';
 
 interface ScheduleMessageModalProps {
   roomId: string;
@@ -65,32 +66,6 @@ function formatSendAt(sendAt: Date): string {
   return `${sendAt.toLocaleDateString()} at ${timeStr}`;
 }
 
-function toLocalDate(date: Date): string {
-  const pad = (n: number) => String(n).padStart(2, '0');
-  return `${date.getFullYear()}-${pad(date.getMonth() + 1)}-${pad(date.getDate())}`;
-}
-
-function toLocalTime(date: Date): string {
-  const pad = (n: number) => String(n).padStart(2, '0');
-  return `${pad(date.getHours())}:${pad(date.getMinutes())}`;
-}
-
-// Shared style for date/time inputs — dark-mode calendar/clock popup via colorScheme.
-const pickerInputStyle = (c: typeof color, cfg: typeof config): React.CSSProperties => ({
-  background: c.SurfaceVariant.Container,
-  color: c.SurfaceVariant.OnContainer,
-  border: `${cfg.borderWidth.B300} solid ${c.SurfaceVariant.ContainerLine}`,
-  borderRadius: cfg.radii.R300,
-  padding: `${cfg.space.S200} ${cfg.space.S300}`,
-  fontSize: '0.875rem',
-  width: '100%',
-  boxSizing: 'border-box',
-  outline: 'none',
-  fontFamily: 'inherit',
-  // Hint browser to render the calendar/clock popup in dark mode
-  colorScheme: 'dark',
-});
-
 export function ScheduleMessageModal({
   roomId,
   initialBody,
@@ -119,11 +94,10 @@ export function ScheduleMessageModal({
   const [dateValue, setDateValue] = useState<string>(() => toLocalDate(def));
   const [timeValue, setTimeValue] = useState<string>(() => toLocalTime(def));
 
-  const getSendAt = useCallback((): Date | null => {
-    if (!dateValue || !timeValue) return null;
-    const dt = new Date(`${dateValue}T${timeValue}:00`);
-    return Number.isNaN(dt.getTime()) ? null : dt;
-  }, [dateValue, timeValue]);
+  const getSendAt = useCallback(
+    (): Date | null => parseLocalDateTime(dateValue, timeValue),
+    [dateValue, timeValue],
+  );
 
   const [preview, setPreview] = useState<{ label: string; relative: string } | null>(null);
 

src/app/features/room/message/RemindMeDialog.tsx

@@ -19,6 +19,12 @@ import {
 import { stopPropagation } from '../../../utils/keyboard';
 import { useReminders } from '../../../hooks/useReminders';
 import { useModalStyle } from '../../../hooks/useModalStyle';
+import {
+  toLocalDate,
+  toLocalTime,
+  parseLocalDateTime,
+  pickerInputStyle,
+} from '../../../utils/datetimeInput';
 
 type RemindMeDialogProps = {
   roomId: string;
@@ -40,14 +46,26 @@ function getPresets(): Array<{ label: string; ms: number }> {
   ];
 }
 
+// Default custom pick: 1 hour from now, rounded up to the nearest 5 minutes.
+function defaultCustomDate(): Date {
+  const d = new Date(Date.now() + 60 * 60 * 1000);
+  d.setSeconds(0, 0);
+  d.setMinutes(Math.ceil(d.getMinutes() / 5) * 5);
+  return d;
+}
+
 export function RemindMeDialog({ roomId, eventId, previewText, onClose }: RemindMeDialogProps) {
   const modalStyle = useModalStyle(320);
   const { addReminder } = useReminders();
   const presets = useMemo(() => getPresets(), []);
   const [busy, setBusy] = useState(false);
   const [error, setError] = useState<string | null>(null);
+  const [customOpen, setCustomOpen] = useState(false);
+  const def = useMemo(() => defaultCustomDate(), []);
+  const [dateValue, setDateValue] = useState(() => toLocalDate(def));
+  const [timeValue, setTimeValue] = useState(() => toLocalTime(def));
 
-  const handlePick = async (ms: number) => {
+  const commit = async (timestamp: number) => {
     if (busy) return;
     setBusy(true);
     setError(null);
@@ -55,7 +73,7 @@ export function RemindMeDialog({ roomId, eventId, previewText, onClose }: Remind
       await addReminder({
         roomId,
         eventId,
-        timestamp: Date.now() + ms,
+        timestamp,
         message: previewText || 'Reminder',
       });
       onClose();
@@ -65,6 +83,23 @@ export function RemindMeDialog({ roomId, eventId, previewText, onClose }: Remind
     }
   };
 
+  const handlePick = (ms: number) => commit(Date.now() + ms);
+
+  const customDate = parseLocalDateTime(dateValue, timeValue);
+  const customValid = !!customDate && customDate.getTime() - Date.now() >= 60_000;
+
+  const handleCustom = () => {
+    if (!customDate) {
+      setError('Please select a valid date and time.');
+      return;
+    }
+    if (customDate.getTime() - Date.now() < 60_000) {
+      setError('Reminder time must be at least 1 minute in the future.');
+      return;
+    }
+    commit(customDate.getTime());
+  };
+
   return (
     <Overlay open backdrop={<OverlayBackdrop />}>
       <OverlayCenter>
@@ -127,6 +162,70 @@ export function RemindMeDialog({ roomId, eventId, previewText, onClose }: Remind
                   </Text>
                 </Button>
               ))}
+
+              {customOpen ? (
+                <Box direction="Column" gap="200" style={{ paddingTop: config.space.S100 }}>
+                  <Box gap="200">
+                    <Box direction="Column" gap="100" style={{ flex: 1 }}>
+                      <Text as="label" htmlFor="remind-date" size="T200" priority="400">
+                        Date
+                      </Text>
+                      <input
+                        id="remind-date"
+                        type="date"
+                        value={dateValue}
+                        min={toLocalDate(new Date())}
+                        disabled={busy}
+                        onChange={(e) => setDateValue(e.target.value)}
+                        style={pickerInputStyle(color, config)}
+                      />
+                    </Box>
+                    <Box direction="Column" gap="100" style={{ flex: 1 }}>
+                      <Text as="label" htmlFor="remind-time" size="T200" priority="400">
+                        Time
+                      </Text>
+                      <input
+                        id="remind-time"
+                        type="time"
+                        value={timeValue}
+                        disabled={busy}
+                        onChange={(e) => setTimeValue(e.target.value)}
+                        style={pickerInputStyle(color, config)}
+                      />
+                    </Box>
+                  </Box>
+                  {!customValid && (dateValue || timeValue) && (
+                    <Text size="T200" style={{ color: color.Critical.Main }}>
+                      Must be at least 1 minute in the future
+                    </Text>
+                  )}
+                  <Button
+                    size="300"
+                    variant="Primary"
+                    radii="300"
+                    disabled={busy || !customValid}
+                    onClick={handleCustom}
+                  >
+                    <Text size="B300">Set reminder</Text>
+                  </Button>
+                </Box>
+              ) : (
+                <Button
+                  size="300"
+                  variant="Secondary"
+                  fill="None"
+                  radii="300"
+                  disabled={busy}
+                  onClick={() => {
+                    setError(null);
+                    setCustomOpen(true);
+                  }}
+                  before={<Icon src={Icons.Clock} size="100" />}
+                >
+                  <Text size="B300">Custom time…</Text>
+                </Button>
+              )}
+
               {error && (
                 <Text
                   size="T200"

src/app/utils/datetimeInput.test.ts

@@ -0,0 +1,45 @@
+import assert from 'node:assert/strict';
+import { test } from 'node:test';
+import { toLocalDate, toLocalTime, parseLocalDateTime } from './datetimeInput';
+
+// All helpers work in the LOCAL timezone. We construct Dates with the local
+// component constructor so the assertions hold regardless of where tests run.
+
+test('toLocalDate: zero-pads month and day', () => {
+  assert.equal(toLocalDate(new Date(2026, 0, 5, 9, 3)), '2026-01-05');
+  assert.equal(toLocalDate(new Date(2026, 11, 25, 0, 0)), '2026-12-25');
+});
+
+test('toLocalTime: zero-pads hours and minutes (24h)', () => {
+  assert.equal(toLocalTime(new Date(2026, 0, 5, 9, 3)), '09:03');
+  assert.equal(toLocalTime(new Date(2026, 0, 5, 23, 59)), '23:59');
+  assert.equal(toLocalTime(new Date(2026, 0, 5, 0, 0)), '00:00');
+});
+
+test('toLocalDate/toLocalTime round-trip through parseLocalDateTime', () => {
+  const d = new Date(2026, 5, 15, 14, 30, 0, 0);
+  const parsed = parseLocalDateTime(toLocalDate(d), toLocalTime(d));
+  assert.ok(parsed);
+  assert.equal(parsed.getTime(), d.getTime());
+});
+
+test('parseLocalDateTime: returns null for missing parts', () => {
+  assert.equal(parseLocalDateTime('', '09:00'), null);
+  assert.equal(parseLocalDateTime('2026-01-05', ''), null);
+  assert.equal(parseLocalDateTime('', ''), null);
+});
+
+test('parseLocalDateTime: returns null for an invalid combination', () => {
+  assert.equal(parseLocalDateTime('not-a-date', '09:00'), null);
+  assert.equal(parseLocalDateTime('2026-13-40', '09:00'), null);
+});
+
+test('parseLocalDateTime: valid input yields the local wall-clock time', () => {
+  const parsed = parseLocalDateTime('2026-01-05', '09:03');
+  assert.ok(parsed);
+  assert.equal(parsed.getFullYear(), 2026);
+  assert.equal(parsed.getMonth(), 0);
+  assert.equal(parsed.getDate(), 5);
+  assert.equal(parsed.getHours(), 9);
+  assert.equal(parsed.getMinutes(), 3);
+});

src/app/utils/datetimeInput.ts

@@ -0,0 +1,45 @@
+import { CSSProperties } from 'react';
+import { color as foldsColor, config as foldsConfig } from 'folds';
+
+const pad = (n: number): string => String(n).padStart(2, '0');
+
+// Format a Date as the value string expected by <input type="date"> in the
+// user's LOCAL timezone. (toISOString would shift to UTC and can land on the
+// wrong calendar day.)
+export function toLocalDate(date: Date): string {
+  return `${date.getFullYear()}-${pad(date.getMonth() + 1)}-${pad(date.getDate())}`;
+}
+
+// Format a Date as the value string expected by <input type="time"> (local, HH:mm).
+export function toLocalTime(date: Date): string {
+  return `${pad(date.getHours())}:${pad(date.getMinutes())}`;
+}
+
+// Parse the local date + time <input> values back into a Date. Returns null when
+// either value is missing or the combination is invalid (e.g. a partial entry).
+export function parseLocalDateTime(dateValue: string, timeValue: string): Date | null {
+  if (!dateValue || !timeValue) return null;
+  const dt = new Date(`${dateValue}T${timeValue}:00`);
+  return Number.isNaN(dt.getTime()) ? null : dt;
+}
+
+// Shared style for date/time <input>s — matches the app's surface tokens and
+// hints a dark-mode calendar/clock popup via colorScheme.
+export function pickerInputStyle(
+  c: typeof foldsColor = foldsColor,
+  cfg: typeof foldsConfig = foldsConfig,
+): CSSProperties {
+  return {
+    background: c.SurfaceVariant.Container,
+    color: c.SurfaceVariant.OnContainer,
+    border: `${cfg.borderWidth.B300} solid ${c.SurfaceVariant.ContainerLine}`,
+    borderRadius: cfg.radii.R300,
+    padding: `${cfg.space.S200} ${cfg.space.S300}`,
+    fontSize: '0.875rem',
+    width: '100%',
+    boxSizing: 'border-box',
+    outline: 'none',
+    fontFamily: 'inherit',
+    colorScheme: 'dark',
+  };
+}