How to write a calendar for Android: from idea to release

Creating your own calendar for the Android operating system is a great task for beginners and experienced developers, allowing you to dive deep into working with dates, local data storage and complex user interfaces. Unlike simple notes, a calendar requires precise mathematical logic to display a grid of days, account for leap years and different time zones. If you want to understand how to write a calendar for Android, you will have to master working with the API java.time or ThreeTenABP, as well as properly design the database.

The modern Android ecosystem offers many tools that simplify this task: from a powerful library Jetpack Compose for layout to architecture Room for data persistence. However, despite the abundance of ready-made solutions, writing your own calendar engine provides a unique understanding of how the system components of the scheduler work. In this article, we will look at the key stages of development, from choosing an architectural pattern to publishing on Google Play.

Selecting a technology stack and application architecture

The first step in development is choosing a technology stack, which will determine the flexibility and performance of your product. For modern projects, the language Kotlin in conjunction with a declarative UI framework is becoming the de facto standard Jetpack Compose. This allows you to create adaptive interfaces that display correctly on both smartphones and tablets with large screens. The use of XML markup is gradually becoming a thing of the past, although legacy code is still found in many corporate projects.

It is important to immediately decide on the architectural approach. The most reliable option is the pattern MVVM (Model-View-ViewModel), which clearly separates display logic, business logic and data. This makes it easier to test and maintain your code in the future. You will need to inject dependencies via Hilt or Kointo manage the lifecycle of objects and avoid memory leaks.

To store events locally, you need to use an abstraction over SQLite. The library Room provides a convenient layer for working with the database, allowing you to write queries in a language similar to SQL, but with compilation at the assembly stage. This is critical for performance, since the calendar must instantly display events even if there are thousands of entries in the database.

• 📱 Development language: Kotlin (preferred) or Java.
• 🎨 UI Toolkit: Jetpack Compose for flexible layout of the day grid.
• 🗄️ Database: Room Persistence Library with support for data streams.
• ⚙️ Architecture: Clean Architecture or MVVM using Coroutines.

⚠️ Attention: When choosing third-party libraries, always check the last update date in the repository. Using abandoned dependencies may lead to incompatibility with new versions of Android and security vulnerabilities.

Event database design

The heart of any calendar is the data structure. You need to create an entity Event, which will describe each event. This table must contain a unique identifier, title, description, start and end times, and recurrence information. Proper design of the data schema will avoid complex queries in the future and ensure quick retrieval of information.

Particular attention should be paid to storing dates. Never store dates as strings of the format "dd.MM.yyyy". Always use a timestamp in milliseconds or ISO-8601 format. This will make it easier to sort events and calculate durations. To work with recurring events (daily, weekly, monthly), you will need a separate table or JSON field inside the main entity that describes the recurrence rules.

Here is an example of what an entity might look like in Room annotations. Pay attention to the use of types Long for time and indexes to speed up searching across date ranges.

@Entity(tableName = "events")
data class Event(
@PrimaryKey(autoGenerate = true) val id: Long = 0,
val title: String,
val description: String?,
val startTime: Long,
val endTime: Long,
val color: Int,
val isAllDay: Boolean = false
)

To work effectively with the database, you need to create a DAO (Data Access Object). It should provide methods for inserting, updating and, most importantly, retrieving events for a certain period. The query should be optimized to select only those records that fall within the user-visible date range.

Field	Data Type	Description	Index
id	Long (Primary Key)	Unique event number	Yes
start_time	Long (Timestamp)	Event start time in ms	Yes
end_time	Long (Timestamp)	Event end time in ms	No
recurrence_rule	String (RRULE)	Repetition rule (RFC 5545)	No

Logic for working with dates and times

Working with a calendar is impossible without a deep understanding of how computers perceive time. On Android, starting with API version 26, it is recommended to use the package java.time. It replaced the outdated and problematic class Calendar. The LocalDate, LocalDateTime and ZonedDateTime classes are immutable, which makes the code more predictable and safe for a multi-threaded environment.

One ​​of the most difficult tasks is displaying the month grid. You need to calculate what day of the week the month starts from and how many days it contains. For this, methods like lengthOfMonth() and getDayOfWeek()are used. Don't forget to take leap years into account when February has 29 days. An error in this calculation will cause the entire calendar grid to shift.

It is also critical to take time zones into account. An event created in Moscow should be displayed correctly for a user located in New York if he changes the device settings. Store all data in UTC, and convert to local time only at the stage of display in the interface. This guarantees data consistency when synchronizing with the server.

To implement the logic for switching between months and years, create a separate manager class, for example CalendarManager. He will be responsible for navigation: “next month”, “previous week”, “today”. This will allow you to separate business logic from UI components.

User interface development

The visual part of the calendar is what the user interacts with most often. Implementing a day grid (Month View) requires the use of components of type LazyVerticalGrid in Jetpack Compose. Each grid element represents a separate day, which can contain event indicators, current day highlighting, and weekend signs.

In addition to the monthly view, it is necessary to implement the Week View and Day Viewmodes. In these modes, the complexity increases, since you need to display events in the form of blocks, the height of which is proportional to the duration of the event. This requires precise coordinate calculations y for each block based on the start and end times.

Don't forget about interactivity. The user should be able to click on a day to see a list of events, or long press to quickly create a new entry. Animations when switching months add a sense of fluidity and quality to the application. Use AnimatedVisibility and animation modifiers to improve UX.

• 📅 Month View: Grid 7 columns, displaying weekday headers.
• ⏳ Day/Week View: Vertical timeline with event blocks.
• 🎨 Theming: Support for dark and light themes via MaterialTheme.
• 🖱️ Gestures: Swipes for navigation between time periods.

⚠️ Attention: When rendering a large number of events in the "Week" or "Month" mode, performance may drop. Use Lazy Loading techniques and do not create unnecessary composition objects inside rendering loops.

How to optimize the rendering of an event list?

Use stable keys for list elements in Compose. This will prevent the list from being completely redrawn when the data of one element changes, which significantly saves processor resources.

Integration with the system calendar and notifications

The native calendar application must be able to interact with the provider's system storage CalendarContract. This allows your application to read events from Google Calendar, Yandex.Calendar and other installed sources. To do this, you need to request the appropriate permissions in the manifest: READ_CALENDAR and WRITE_CALENDAR.

Starting with Android 6.0 (API 23), permissions are requested at runtime. You must handle the scenario gracefully when a user is denied access. In this case, the application should work in a limited mode, using only the local database, but not show errors or crashes.

For event reminders, you must use AlarmManager or WorkManager. AlarmManager suitable for precise triggering at a specific time (for example, 10 minutes before a meeting), whereas WorkManager is best used for background data synchronization. Don't forget to create Notification Channels for Android 8.0 and higher so that the user can flexibly customize the sounds and importance of notifications.

// Example of creating an event notification
val notificationManager = getSystemService(NOTIFICATION_SERVICE) as NotificationManager
val notification = NotificationCompat.Builder(context, CHANNEL_ID)
.setSmallIcon(R.drawable.ic_event)
.setContentTitle(event.title)
.setContentText("Starts in 15 minutes")
.setPriority(NotificationCompat.PRIORITY_HIGH)
.build()
notificationManager.notify(event.id.toInt(), notification)

Testing, Debugging, and Publishing

The final stage of development involves extensive testing. A calendar is an application where errors in date logic may only appear after a year or under specific conditions (for example, daylight saving time in certain countries). Be sure to write unit tests (Unit Tests) for date calculation logic and interface testing tools (UI Tests) using Espresso or Compose Testing.

Test the application works on devices with different screen sizes and OS versions. Pay special attention to the behavior when the user changes the system time. The application should not “break” or show incorrect data if the user manually sets the clock back or forward.

Before publishing on Google Play, prepare high-quality screenshots, a description and a privacy policy. If your app requests access to contacts or calendar, you will need to justify the need for these permissions during moderation. Google strictly ensures that permission requests correspond to the functionality of the application.

• 🧪 Unit tests: Checking the logic of leap years and time zones.
• 📲 UI tests: Automation of scripts for creating and deleting events.
• 🌍 Localization: Support for different languages and date formats.
• 🔒 Security: Encryption of sensitive data in the database.

⚠️ Attention: Google Play rules regarding permissions change frequently. Before publishing, be sure to check with the official developer help center to ensure that your application is not rejected due to an unreasonable request for access to the calendar.

Frequently asked questions (FAQ)

Which library is best to use to work with dates?

For modern projects (minSdk 26+) use the built-in package java.time. If you need support for very old devices (below API 26), connect the library ThreeTenABP, which is a port of java.time for older versions of Android. Avoid the legacy class Calendar.

Do you need to request Internet access permission for the calendar?

No, if your application only works with the local database and system calendar provider, Internet access is not required. Permission is only needed if you plan to synchronize with your own server or third-party APIs.

How to implement a dark theme in the calendar?

Use color resources with suffixes -night in the values ​​folders. In Jetpack Compose, this is done through MaterialThemewhere you define a color scheme darkColors() and apply it depending on system settings or user choice.

Why do events shift when changing time zones?

You are most likely storing the time in a local format instead UTC. Always convert the user's time to UTC before saving to the database and back to local time when displaying, taking into account the current TimeZone device.

Can I use a ready-made calendar widget?

There are third-party libraries that provide ready-made Views for the calendar, but for a full-fledged application with a unique design and logic, it is better to write your own components. This will give complete control over performance and appearance.