Design a multi-device calendar application

## System design prompt Design a calendar application similar to Apple Calendar. ### Core features - Users can **create, update, and delete events**. - The UI must support **Day / Month / Year views** (i.e., the backend must efficiently fetch events for a given time range). - **Multi-device support**: if a user creates/edits an event on one device, it should **sync to the user’s other devices**. ### Clarifications you should cover - Assumptions about data scale (users, events per user, read/write QPS). - Consistency expectations for cross-device sync (e.g., eventual vs “near real-time”). - Handling tricky calendar cases (time zones, all-day events, recurring events, edits while offline, conflicts). ### Output expected from candidate - High-level architecture (clients, APIs, storage, background jobs, push/sync). - Data model and key indexes. - Sync protocol and conflict-resolution strategy. - How to serve Day/Month/Year queries efficiently. - Key tradeoffs and failure/edge cases.