### ANSWER

## 1. Core Functionality

A calendar app needs clear day, week, and month views; fast event creation and editing; recurring events; reminders such as 10 minutes before a meeting; and support for a 12/24-hour clock.

Two-way synchronization should keep changes consistent across two devices while handling conflicts, offline edits, time zones, invitations, shared calendars, search, accessibility, privacy, and dependable notifications.
