Best open-source alternatives to Google Calendar

Nextcloud Server is a self-hosted collaboration and file platform for individuals, teams, and organizations that want control over their data. It provides a central place to store files and also supports contacts, calendars, communication features, and sharing across devices. The project is designed for users who want cloud-like functionality on infrastructure they control, whether at home or in a company environment. It is extensible through a large app ecosystem, includes security features such as encryption and two-factor authentication, and can be deployed by self-installation, through hosting providers, or via appliances and preinstalled devices.

Radicale is a lightweight CalDAV and CardDAV server designed to store and share calendars, contacts, and related data over standard protocols. It is aimed at users and administrators who want a simple self-hosted groupware backend that works with many CalDAV and CardDAV clients without requiring a complicated setup. The project emphasizes ease of deployment, file-based storage, and extensibility. It supports calendars, to-do lists, journal entries, and business cards, and it can restrict access with authentication and secure connections with TLS. Because all data is kept on the filesystem in a simple folder structure, it is suitable for small deployments and for users who prefer straightforward backup and maintenance workflows.

Baïkal is a lightweight server project for running CalDAV and CardDAV services. It is intended for people who want to host their own calendar and contact synchronization backend, such as for use with clients like Thunderbird, Android apps, DavX5, and iOS devices. The README mainly points users to the project website for installation, upgrade, and troubleshooting guidance, and it also references community-written step-by-step tutorials in German and French. The repository itself is described as the source repository, and the project is developed by volunteers. The README does not describe the internal architecture or configuration details beyond its purpose as a calendar and address book server.

SOGo is presented as a collaborative free and open source groupware project. The README focuses on how contributors can help improve the software through documentation, translations, feature ideas, discussion, and code patches, rather than describing end-user workflows in detail. It also explains how to obtain the source code for SOGo and its related components, including SOPE and the Thunderbird connector extension, and points readers to compilation instructions in the project FAQ. The document further highlights the software’s internationalization support by listing a broad set of official translations and maintainers, suggesting a mature, community-driven project with a strong emphasis on multilingual use and contribution.

Xandikos is a small CalDAV and CardDAV server designed to back calendar and address book data with a Git repository. It is aimed at users who want a self-hosted DAV service that can integrate with standard calendaring and contacts clients while keeping data in a familiar, versioned storage model. The project implements a broad set of WebDAV, CalDAV, and CardDAV standards, including scheduling-related extensions and selected WebDAV push and iMIP features. It can run directly on a plain HTTP socket or behind a reverse proxy, and it supports containerized deployment as well as installation from Python/package dependencies. The README also notes tested compatibility with many desktop and mobile clients, and mentions that multi-user support is experimental.

EGroupware is a web-based groupware system intended for teams and organizations that need shared productivity tools. The README focuses on how to install and maintain the platform rather than describing end-user features in detail, but it makes clear that the project is designed to run as a multi-container application and that a package-manager-based deployment is the preferred approach for most users. It supports multiple deployment paths, including Linux packages, Docker for non-Linux environments, and a source-based development setup. The development workflow mentions optional app repositories, installer scripts, and update channels, which suggests a modular system with separate components and maintenance routines. The project is aimed at administrators and developers who need to deploy, extend, and keep a self-hosted groupware installation up to date.

Davis is a self-hosted DAV server project that combines a backend service, an administrative web interface, and a frontend for managing calendars and address books. It is built on sabre/dav with Symfony 7 and Bootstrap 5, and is presented as a modern replacement-style solution inspired by Baïkal. The project appears aimed at users or administrators who need CalDAV, CardDAV, and WebDAV services with straightforward browser-based management. Its dashboard supports user editing, calendar creation and sharing, address book creation, public calendars, birthday calendars, and notifications, while the server layer adds scheduling and mail-related features depending on client support. Deployment is designed to be flexible: the README describes local installation via Composer, production setup with migrations, and containerized deployment using Docker or docker-compose. It also documents configuration through environment variables for authentication, database access, mail delivery, and WebDAV paths, making it suitable for self-hosted environments and administrators who want direct control over the service.

Manage my Damn Life (MMDL) is a self-hosted web frontend for managing CalDAV tasks and calendar events. It is aimed at users who want a desktop-first interface for interacting with their CalDAV data, including task lists, calendar views, and Gantt-style planning. The project supports multiple CalDAV accounts and multiple user accounts, and it has been tested with Nextcloud and Baikal. It is described as being in beta, and the README notes that current CalDAV authentication support is basic authentication, with OAuth support documented separately. Translation management is handled through Weblate.

Penguin Mail is a desktop mail and calendar application for Linux, implemented in Rust with GTK and libadwaita. It is designed for people who want a unified inbox and calendar experience across Gmail, Microsoft accounts, and standard IMAP/POP3 providers, while keeping mail stored on the user’s own computer. The project combines everyday email workflows with a broad set of productivity features, including conversation threading, category views, search, smart mailboxes, reminders, rules, automatic replies, and export options. It also emphasizes privacy and security through OpenPGP and S/MIME support, blocked remote content, and an assistant pane that can summarize, draft replies, and interact with calendars using either local models or external AI services.

Keeper.sh is a self-hostable calendar synchronization system designed for people who need to keep several calendars aligned across providers. It can ingest events from Google Calendar, Outlook, iCloud, Fastmail, CalDAV servers, and remote iCal/ICS links, then push them to one or many destination calendars so schedules stay consistent. The project is aimed at users who want more control over calendar data, including self-hosters and technically oriented teams. It provides a REST API and MCP server for automation, incremental syncing for some providers, privacy controls for event contents, and a combined iCal feed. The README also distinguishes between cloud-hosted and self-hosted usage, with self-hosting including Pro features by default.

FocusFlow is a self-hosted Pomodoro time-management system made up of a Rust backend and a native client built with Tauri and SvelteKit. It is aimed at people who want to organize work and break sessions, manage tasks, and review productivity data across devices. The project includes cloud infrastructure, a backend API, and a native app for macOS, Windows, Linux, and Android. It supports real-time synchronization through WebSockets, provides a documented REST API, and offers self-hosted deployment through Docker Compose or Kubernetes. The client connects to a user-supplied backend URL on first launch, making it suitable for personal or privately hosted use.

Server for calendar sharing (CalDAV) that uses a PostgreSQL database as a data store.