Implementing Calendar and Planner in Mobile App
Embedding "just calendar" is one of those wishes transforming into two-week task. Time zones, date localization, recurring events, system calendar sync—each point adds non-trivial complexity.
Architectural Solutions
Use Ready Library or Write from Scratch
For most apps—ready library. On iOS: FSCalendar—one of most mature, supports customization via appearance API and delegate methods, works on UIKit. For SwiftUI—swift-calendar or own implementation via LazyVGrid with Calendar API.
On Android: kizitonwose/calendar-library (CalendarView and WeekCalendarView)—well documented, Compose-compatible. Material Design 3 contains DatePicker and DateRangePicker, but only for date selection, not event display.
In Flutter: table_calendar—de-facto standard with 2000+ stars, supports event markers, locale, display formats.
Writing from scratch justified only with very non-standard design or specific performance needs with thousands of monthly events.
Working with Dates and Time Zones
Most frequent bug: event created in Moscow (UTC+3), displays on wrong day for user in Berlin (UTC+2). Reason—storing and comparing dates without time zone accounting.
Rule: always transmit UTC timestamp to API (ISO 8601: 2024-03-15T14:30:00Z), convert to local time only for display. On iOS—TimeZone.current on Calendar initialization and DateFormatter. Calendar(identifier: .gregorian) with calendar.timeZone = TimeZone(identifier: "Europe/Moscow")! for fixed-zone events (e.g., conference in Moscow displays in Moscow time for everyone).
On Android—java.time.ZonedDateTime (API 26+) or ThreeTenBP library for older. LocalDate.ofInstant(instant, ZoneId.of("Europe/Berlin"))—safe conversion. Date and Calendar from java.util considered obsolete; if legacy code uses them—plan migration.
Recurring Events
Recurring events (recurrence rules)—RFC 5545 standard (iCalendar). RRULE:FREQ=WEEKLY;BYDAY=MO,WE,FR;UNTIL=20241231T235959Z—every Monday, Wednesday, Friday until end of 2024.
On iOS—EventKit framework fully supports RRULE via EKRecurrenceRule. Integrating with system calendar—use EKEventStore and EKEvent.recurrenceRules. For custom storage—need RRULE parser. Ready implementation: ical4j on JVM, rrule.js for JavaScript, on Swift—small open libraries like RRuleSwift.
Don't store each recurring event instance—only rule and exceptions (EXDATE). Generate instances on-the-fly for display date range.
System Calendar Sync
iOS—EventKit with EKEntityType.event permission. Create, read, update events in system calendar via EKEventStore. User can choose which calendar—show UIAlertController with list from eventStore.calendars(for: .event).
Android—CalendarProvider ContentProvider. Access via ContentResolver with CalendarContract.Events.CONTENT_URI URI. Requires READ_CALENDAR and WRITE_CALENDAR permissions. More complex than EventKit, similar capabilities.
Important: bidirectional sync. If user deleted event in system calendar—app must detect. EKEventStore.reset() invalidates cached data; listen EKEventStoreChangedNotification.
Performance with Many Events
FSCalendar and similar handle hundreds fine. Thousands need virtualization: load events only for visible month + 1 month forward/back. NSFetchedResultsController on iOS or Room + Flow on Android for reactive updates on data change.
Event markers (dots under date)—don't render UIView per dot. CALayer or custom drawRect: with UIGraphicsGetCurrentContext() thousands times faster for cells with 5+ events.
Display Types
Monthly, weekly, daily—each needs separate layout. Switch via UIPageViewController or horizontal UICollectionView with paging. Smooth transition: crossDissolve animation on mode change, save selected date as anchor.
Timeline: basic monthly calendar with events—1 week. Full planner with weekly/daily view, recurring events, system calendar sync and offline support—2 weeks.