Recipient header date format

assets/l10n/app_en.arb

@@ -364,6 +364,14 @@
   "@errorMarkAsReadFailedTitle": {
     "description": "Error title when mark as read action failed."
   },
+  "today": "Today",
+  "@today": {
+    "description": "Term to use to reference the current day."
+  },
+  "yesterday": "Yesterday",
+  "@yesterday": {
+    "description": "Term to use to reference the previous day."
+  },
   "userRoleOwner": "Owner",
   "@userRoleOwner": {
     "description": "Label for UserRole.owner"

lib/widgets/message_list.dart

@@ -710,6 +710,7 @@ class RecipientHeaderDate extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
+    final zulipLocalizations = ZulipLocalizations.of(context);
     return Padding(
       padding: const EdgeInsets.fromLTRB(10, 0, 16, 0),
       child: Text(
@@ -725,12 +726,48 @@ class RecipientHeaderDate extends StatelessWidget {
           //   https://developer.mozilla.org/en-US/docs/Web/CSS/font-variant-caps#all-small-caps
           fontFeatures: const [FontFeature.enable('c2sc'), FontFeature.enable('smcp')],
         ).merge(weightVariableTextStyle(context)),
-        _kRecipientHeaderDateFormat.format(
-          DateTime.fromMillisecondsSinceEpoch(message.timestamp * 1000))));
+        formatHeaderDate(
+          zulipLocalizations,
+          DateTime.fromMillisecondsSinceEpoch(message.timestamp * 1000),
+          now: DateTime.now())));
   }
 }
 
-final _kRecipientHeaderDateFormat = DateFormat('y-MM-dd', 'en_US'); // TODO(#278)
+@visibleForTesting
+String formatHeaderDate(
+  ZulipLocalizations zulipLocalizations,
+  DateTime dateTime, {
+  required DateTime now,
+}) {
+  assert(!dateTime.isUtc && !now.isUtc,
+    '`dateTime` and `now` need to be in local time.');
+
+  if (dateTime.year == now.year &&
+      dateTime.month == now.month &&
+      dateTime.day == now.day) {
+    return zulipLocalizations.today;
+  }
+
+  final yesterday = now
+    .copyWith(hour: 12, minute: 0, second: 0, millisecond: 0, microsecond: 0)
+    .add(const Duration(days: -1));
+  if (dateTime.year == yesterday.year &&
+      dateTime.month == yesterday.month &&
+      dateTime.day == yesterday.day) {
+    return zulipLocalizations.yesterday;
+  }
+
+  // If it is Dec 1 and you see a label that says `Dec 2`
+  // it could be misinterpreted as Dec 2 of the previous
+  // year. For times in the future, those still on the
+  // current day will show as today (handled above) and
+  // any dates beyond that show up with the year.
+  if (dateTime.year == now.year && dateTime.isBefore(now)) {
+    return DateFormat.MMMd().format(dateTime);
+  } else {
+    return DateFormat.yMMMd().format(dateTime);
+  }
+}
 
 /// A Zulip message, showing the sender's name and avatar if specified.
 class MessageWithPossibleSender extends StatelessWidget {

test/widgets/message_list_test.dart

@@ -366,7 +366,7 @@ void main() {
     testWidgets('show dates', (tester) async {
       await setupMessageListPage(tester, messages: [
         eg.streamMessage(timestamp: 1671409088),
-        eg.dmMessage(timestamp: 1692755322, from: eg.selfUser, to: []),
+        eg.dmMessage(timestamp: 1661219322, from: eg.selfUser, to: []),
       ]);
       // We show the dates in the user's timezone.  Dart's standard library
       // doesn't give us a way to control which timezone is used — only to
@@ -377,11 +377,34 @@ void main() {
       //   https://github.com/dart-lang/sdk/issues/28985 (about DateTime.now, not timezone)
       //   https://github.com/dart-lang/sdk/issues/44928 (about the Dart implementation's own internal tests)
       // For this test, just accept outputs corresponding to any possible timezone.
-      tester.widget(find.textContaining(RegExp("2022-12-1[89]")));
-      tester.widget(find.textContaining(RegExp("2023-08-2[23]")));
+      tester.widget(find.textContaining(RegExp("Dec 1[89], 2022")));
+      tester.widget(find.textContaining(RegExp("Aug 2[23], 2022")));
     });
   });
 
+  group('formatHeaderDate', () {
+    final zulipLocalizations = GlobalLocalizations.zulipLocalizations;
+    final now = DateTime.parse("2023-01-10 12:00");
+    final testCases = [
+      ("2023-01-10 12:00", zulipLocalizations.today),
+      ("2023-01-10 00:00", zulipLocalizations.today),
+      ("2023-01-10 23:59", zulipLocalizations.today),
+      ("2023-01-09 23:59", zulipLocalizations.yesterday),
+      ("2023-01-09 00:00", zulipLocalizations.yesterday),
+      ("2023-01-08 00:00", "Jan 8"),
+      ("2022-12-31 00:00", "Dec 31, 2022"),
+      // Future times
+      ("2023-01-10 19:00", zulipLocalizations.today),
+      ("2023-01-11 00:00", "Jan 11, 2023"),
+    ];
+    for (final (dateTime, expected) in testCases) {
+      test('$dateTime returns $expected', () {
+        check(formatHeaderDate(zulipLocalizations, DateTime.parse(dateTime), now: now))
+          .equals(expected);
+      });
+    }
+  });
+
   group('MessageWithPossibleSender', () {
     testWidgets('Updates avatar on RealmUserUpdateEvent', (tester) async {
       addTearDown(testBinding.reset);