Initial date planner app (#2390)

Initial **experimental** date planner iOS sample.

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-devrel
channel on [Discord].

<!-- Links -->
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Style-guide-for-Flutter-repo.md
[CLA]: https://cla.developers.google.com/
[Discord]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Chat.md
[Contributors Guide]:
https://github.com/flutter/samples/blob/main/CONTRIBUTING.md

experimental/date_planner/lib/event.dart

@@ -0,0 +1,80 @@
+// Copyright 2024 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/cupertino.dart';
+import 'package:intl/intl.dart';
+import 'package:uuid/uuid.dart';
+
+import 'color_options.dart';
+import 'event_task.dart';
+
+class Event implements Comparable<Event> {
+  static const _uuid = Uuid();
+
+  final id = _uuid.v4();
+  String title;
+  ColorOptions color;
+  IconData icon;
+  List<EventTask> tasks;
+  DateTime date;
+
+  Event({
+    required this.title,
+    ColorOptions? color,
+    this.icon = CupertinoIcons.add,
+    List<EventTask>? tasks,
+    DateTime? date,
+  })  : color = color ?? ColorOptions.random(),
+        tasks = tasks ?? [EventTask(text: '')],
+        date = date ?? DateTime.now();
+
+  Event copy() {
+    return Event(
+      title: title,
+      color: color,
+      icon: icon,
+      tasks: tasks,
+      date: date,
+    );
+  }
+
+  updateWith(Event e) {
+    title = e.title;
+    color = e.color;
+    icon = e.icon;
+    tasks = e.tasks;
+    date = e.date;
+  }
+
+  int get remainingTaskCount => tasks.where((e) => !e.isCompleted).length;
+
+  bool get isComplete => remainingTaskCount == 0;
+
+  bool get isPast => DateTime.now().isAfter(date);
+
+  bool get isWithinSevenDays => !isPast && date.isBefore(FromNow.sevenDays);
+
+  bool get isWithinSevenToThirtyDays =>
+      !isPast && !isWithinSevenDays && date.isBefore(FromNow.thirtyDays);
+
+  bool get isDistant => date.isAfter(FromNow.thirtyDays);
+
+  String get dateFormated => '${DateFormat.yMMMd().format(date)} at '
+      '${DateFormat.Hm().format(date)}';
+
+  @override
+  int compareTo(Event other) => date.compareTo(other.date);
+}
+
+class FromNow {
+  static DateTime get sevenDays => DateTime.now().add(const Duration(days: 7));
+
+  static DateTime get thirtyDays =>
+      DateTime.now().add(const Duration(days: 30));
+
+  static DateTime roundedHours(int hours) {
+    final date = DateTime.now().add(Duration(hours: hours));
+    return DateTime(date.year, date.month, date.day, date.hour);
+  }
+}