# What a Calendar App Needs

A calendar app lets users create, view, and manage scheduled events over time. This document outlines the core requirements across functionality, data, user interface, and system concerns.

## 1. Core Functionality

### Event Management
- **Create events** with, at minimum: title, start date/time, and end date/time.
- **Edit and delete** existing events.
- **All-day events** that span a date (or range of dates) without a specific time.
- **Multi-day events** that cross date boundaries.
- **Time zones**, both for the user's local zone and per-event zones (critical for travel and remote teams).
- **Recurrence rules** for repeating events (daily, weekly, monthly, yearly, custom intervals), including the ability to edit or delete a single occurrence versus the entire series.

### Reminders and Notifications
- Configurable alerts before (or after) an event starts (e.g., 10 minutes, 1 day).
- Local notifications when the app is closed and push notifications for shared/updated events.
- Snooze and dismiss actions.

## 2. Data Requirements

A calendar app needs a reliable data model. Typical entities include:

| Entity | Key Fields |
|---|---|
| **Event** | ID, title, description, location, start/end time, time zone, all-day flag, recurrence rule, organizer |
| **Attendee** | Name, email, RSVP status (accepted, declined, tentative) |
| **Calendar** | Name, color, visibility, owner, sharing permissions |
| **Reminder** | Offset, method (alert, email) |

Additional data considerations:
- **Storage strategy**: local database (e.g., SQLite, Core Data, Room) plus cloud sync.
- **Conflict resolution** when the same event changes on two devices offline.
- **Data persistence** across sessions, app updates, and device changes.

## 3. User Interface Requirements

### Views
- **Day view** — a single day's schedule with time slots.
- **Week view** — the default for most users; shows overlapping events side by side.
- **Month view** — a compact overview with event indicators.
- **Agenda/List view** — a scrollable list of upcoming events.
- **Year view** (optional) — high-level navigation.

### Interaction
- Tap to view event details; long-press or drag to create events at a specific time slot.
- Drag-and-drop rescheduling and resize-to-adjust duration.
- Search by event title, location, or attendee.
- "Go to today" navigation and smooth scrolling/swiping between periods.

## 4. Synchronization and Integration

- **Cross-device sync** so events appear consistently on phone, tablet, web, and desktop.
- **Standards support** for interoperability:
  - **iCalendar (ICS / RFC 5545)** for import/export and event sharing.
  - **CalDAV** for syncing with external calendar servers.
  - Integration with major providers (Google Calendar, Outlook/Exchange, Apple Calendar).
- **Calendar subscription** to external feeds (e.g., holidays, sports schedules).
- **Accounts** for sign-in and multi-calendar support (personal, work, shared).

## 5. Sharing and Collaboration

- Invite attendees to events and track RSVPs.
- Share entire calendars with view or edit permissions.
- Free/busy lookup to find times when all attendees are available.
- Conflict detection when a new event overlaps an existing one.

## 6. System and Quality Requirements

- **Performance**: fast startup, smooth scrolling in dense months, efficient rendering of large event sets.
- **Offline support**: full read access offline; queued writes that sync when reconnected.
- **Reliability**: alarms fire on time even after device restart; no lost events.
- **Security and privacy**:
  - Encryption in transit (TLS) and at rest.
  - Granular permissions (OS calendar access prompts).
  - Compliance with privacy regulations (GDPR, etc.) for user data.
- **Accessibility**: screen reader support, sufficient color contrast (don't rely on color alone for calendars), keyboard navigation, dynamic text sizing.
- **Localization**: translated labels, locale-aware date/time formats, first day of week (Sunday vs. Monday), and 12/24-hour clock preferences.

## 7. Nice-to-Have Features

- Natural language input ("Lunch with Sam tomorrow at noon").
- Suggested meeting times based on attendee availability.
- Weather display for upcoming events.
- Travel time estimates and location maps.
- Widgets and watch/companion apps.
- Task/to-do integration alongside events.

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**In short:** a calendar app needs robust event and recurrence handling, accurate time zone support, dependable reminders, a fast multi-view UI, reliable cross-device sync built on open standards, secure storage, and accessible, localized design.
