12 Best Open Source Calendar Apps in 2026 (Google Alternatives)
The best open source calendar apps and self-hosted calendar servers in 2026, and how to replace Google Calendar with open source, step by step.
Open Sunsama Team · · 12 min read
The best open source calendar app for planning your day is Open Sunsama. It puts your tasks and your Google, Outlook or iCloud calendar in one day view, with time blocking and any AI agent. It plans on top of the calendar you already have, so it does not replace a calendar server.
To host the calendar itself, pick Nextcloud Calendar for a full self-hosted Google Calendar, or Radicale for the lightest server. Then add Thunderbird on desktop and Fossify Calendar with DAVx⁵ on Android.
We build Open Sunsama, so we put it first — here is why, and where others fit better.
A calendar has two halves: the server where your events live, and the app you look at. Google gives you both, so leaving Google means picking one of each.
Key takeaways
- Best overall: Open Sunsama, the open-source calendar planner. Your tasks and your calendar in one day view, with time blocks and any AI agent.
- Best self-hosted Google Calendar: Nextcloud Calendar, a web calendar with sharing, invites and booking.
- Lightest calendar server: Radicale. Baïkal if your host runs PHP.
- Best desktop app: Thunderbird on Windows, Mac and Linux. GNOME Calendar on Linux.
- Best Android setup: DAVx⁵ to sync, plus Fossify Calendar or Etar to view.
- Encrypted, but hosted by someone else: Proton Calendar and Tuta Calendar. Open apps, closed servers.
- Booking links: Cal.diy, the community edition of Cal.com.
| App | License | Stars | Latest release | Platforms | Self-host | API | MCP |
|---|
| Open Sunsama | Open Sunsama Non-Commercial License | 65 | v1.0.16 (Sep 2026) | Web, macOS, Windows, Linux, Server | Docker | REST | Official |
| Nextcloud Calendar | AGPL-3.0-or-later | 1.2k | v6.6.1 (Sep 2026) | Web, Server | Nextcloud app | CalDAV | Official |
| Radicale | GPL-3.0-or-later | 5k | v3.8.1 (Sep 2026) | Server | Docker, pip | CalDAV, CardDAV, Plugins | Community |
| Baïkal | GPL-3.0-only | 3.3k | 0.12.1 (Aug 2026) | Server | PHP app | CalDAV, CardDAV | Community |
| Thunderbird | MPL-2.0 | 164 | 156.0.1 (Sep 2026) | macOS, Windows, Linux | No | CalDAV, Plugins | Community |
| Fossify Calendar | GPL-3.0-only | 2.2k | 1.11.0 (Sep 2026) | Android | No | None | None found |
| DAVx⁵ | GPL-3.0-only | 2.9k | v4.5.19-ose (Aug 2026) | Android | No | CalDAV, CardDAV, WebDAV | Official |
| GNOME Calendar | GPL-3.0-or-later | 128 | 51.0 (Sep 2026) | Linux | No | CalDAV | Community |
| Etar | GPL-3.0-only AND Apache-2.0 | 2.6k | v1.0.57 (Jul 2026) | Android | No | None | None found |
| Proton Calendar | GPL-3.0-only | 5.6k | 5.0.134.1 (Sep 2026) | Web, iOS, Android | No | None | Community |
| Tuta Calendar | GPL-3.0-only | 7.9k | 360.260922.0 (Sep 2026) | Web, macOS, Windows, Linux, iOS, Android | No | None | None found |
| Cal.diy (Cal.com community edition) | MIT | 48.6k | v6.2.0 (Mar 2026) | Web, Server | Docker, Node.js | REST, Webhooks | Community |
Stars, licenses and releases from each project's repository, checked Sep 2026. App names link to the source code.A calendar server stores events, and calendar apps show them
Most "open source calendar" lists mix up two kinds of software. That is why the same list can hold Radicale and Thunderbird, which do very different jobs.
A calendar server is where your events live. Nextcloud Calendar, Radicale and Baïkal are servers you run yourself. Your phone and laptop sync with it over CalDAV, the open standard for calendar sync.
A calendar app is what you look at. Thunderbird, GNOME Calendar, Fossify Calendar and Etar are apps. They can show events from Google, from your own server, or from both at once.
Some tools sit on top of both. Open Sunsama is a planner that shows your calendar next to your tasks. Cal.diy is a booking page that reads your calendar to find free times.
How we compared
We read each project's license file, releases, docs and open issues. We pulled stars and releases from GitHub, GitLab and Codeberg on September 24, 2026. Each card below shows those facts, with a link to the source code.
We captured the screenshots ourselves from each app's demo or web app, or took them from the project's official images. The card says which.
We build Open Sunsama and use it every day. We did not run every other app for weeks, so we don't claim to have tested them.
1. Open Sunsama: the best open-source calendar planner
1. Open Sunsama
A daily planner with a kanban board of days, time blocking, focus mode and a hosted MCP server for AI agents.
Best for: Planning your day with tasks and your Google, Outlook or iCloud calendar in one view, with time blocks and any AI agent.
Note: The code is public under a non-commercial license: you can read, run and self-host it for personal use. Companies need a commercial license.
Captured from the Open Sunsama web app- License
- Open Sunsama Non-Commercial License
- Stars
- 65 (as of Sep 2026)
- Latest release
- v1.0.16 (Sep 2026)
- Language
- TypeScript
- Platforms
- Web, macOS, Windows, Linux, Server
- Self-host
- Docker
- API
- REST
- MCP
- Official
Source code · Website · Docs · Facts checked Sep 2026
Verdict: best overall. It turns your calendar into a plan for the day, and Claude or ChatGPT can help you make it.
Open Sunsama is a daily planner, not a calendar server. You connect the calendars you already use: Google, Outlook and iCloud, and as many accounts as you like. Your meetings then sit next to your tasks.
The week view shows your time blocks next to your real meetings.
You plan by dragging a task into a gap between meetings. Resize the block to fit the work. Now your to-do list has a time, not just a date.
Drag a task onto the calendar, then resize the block to fit the work.
Sync goes both ways for events. Create, move or resize a meeting in Open Sunsama, and it saves back to Google, Outlook or iCloud. Your time blocks stay in Open Sunsama, so coworkers don't see your plan.
The day view puts today's tasks beside today's calendar.
- Good: time blocking in day and week views, rollover of unfinished tasks, a focus timer that tracks planned vs actual time, and a hosted MCP server. Your agent can read your meetings and plan around them. See calendar sync.
- Falls short: it does not store your calendar or replace a CalDAV server. It reads Google, Outlook and iCloud today. There is no month view, and no iOS or Android app; phones use the mobile web app.
- Pick it if you want one place to see your meetings and plan your tasks around them. Self-host it with Docker, or read why we call it an open source task manager.
2. Nextcloud Calendar: the fullest self-hosted Google Calendar
2. Nextcloud Calendar
The calendar app for Nextcloud, with shared calendars, invites, free/busy and appointment booking.
Best for: People who want a full web calendar with sharing, invites and booking on their own server.
Note: It runs inside a Nextcloud server, so you need one first.
Official screenshot from github.com/nextcloud/calendar, © the Nextcloud Calendar project- License
- AGPL-3.0-or-later (GNU Affero General Public License v3.0 or later)
- Stars
- 1.2k (as of Sep 2026)
- Latest release
- v6.6.1 (Sep 2026)
- Language
- JavaScript
- Platforms
- Web, Server
- Self-host
- Nextcloud app
- API
- CalDAV
- MCP
- Official
Source code · Website · Docs · Facts checked Sep 2026
Verdict: the closest thing to Google Calendar you can host yourself.
- Good: a real web calendar, shared calendars, invites, a "Find a time" free/busy view and public booking pages. It imports your Google .ics files in a few clicks.
- Falls short: you run a whole Nextcloud server to get it. That is more to update and back up than a small CalDAV server.
- Pick it if you already run Nextcloud for files, or you want one server for your whole family.
3. Radicale: the lightest calendar server
3. Radicale
A small CalDAV and CardDAV server that stores your calendars and contacts as plain files.
Best for: People who only need a small sync server for the calendar apps they already use.
Note: It is only a sync server; its web page manages calendars but can't show or edit events.
Captured from a local install of Radicale 3.8.0- License
- GPL-3.0-or-later (GNU General Public License v3.0 or later)
- Stars
- 5k (as of Sep 2026)
- Latest release
- v3.8.1 (Sep 2026)
- Language
- Python
- Platforms
- Server
- Self-host
- Docker, pip
- API
- CalDAV, CardDAV, Plugins
- MCP
- Community
Source code · Website · Docs · Facts checked Sep 2026
Verdict: the top answer on r/selfhosted for a reason. It is small, simple and stores events as plain files.
- Good: one Python install or a Docker image, with almost nothing to set up. It syncs contacts too, over CardDAV.
- Falls short: its web page only creates and manages calendars. You can't see or edit events there, so you need an app on every device.
- Pick it if you want your calendar off Google with the least work and the fewest moving parts.
4. Baïkal: a CalDAV server for PHP hosts
4. Baïkal
A lightweight CalDAV and CardDAV server in PHP, with a web admin to manage users and calendars.
Best for: People with a PHP web host who want a simple calendar and contacts server.
Note: It is only a sync server; you view and edit events in another calendar app.
Official screenshot from sabre.io, © the Baïkal project- License
- GPL-3.0-only (GNU General Public License v3.0 only)
- Stars
- 3.3k (as of Sep 2026)
- Latest release
- 0.12.1 (Aug 2026)
- Language
- PHP
- Platforms
- Server
- Self-host
- PHP app
- API
- CalDAV, CardDAV
- MCP
- Community
Source code · Website · Docs · Facts checked Sep 2026
Verdict: Radicale's job, with a web admin for users and calendars.
- Good: it runs on almost any cheap PHP host. The admin page makes it easy to add family members.
- Falls short: like Radicale, it has no calendar view of its own. There is no official Docker image, only community ones.
- Pick it if you have a PHP host and want a web page to manage users.
5. Thunderbird: the best desktop calendar app
5. Thunderbird
A desktop email app with a built-in calendar and task list that syncs over CalDAV.
Best for: Desktop users on Windows, Mac or Linux who want mail and calendar in one app.
Official screenshot from thunderbird.net, © the Thunderbird project- License
- MPL-2.0 (Mozilla Public License 2.0)
- Stars
- 164 (as of Sep 2026)
- Latest release
- 156.0.1 (Sep 2026)
- Language
- JavaScript
- Platforms
- macOS, Windows, Linux
- Self-host
- No server
- API
- CalDAV, Plugins
- MCP
- Community
Source code · Website · Docs · Facts checked Sep 2026
Verdict: the most complete open source calendar app for your computer.
- Good: it syncs with Google, any CalDAV server and local files, side by side. It can import Google's .ics export straight into your new server.
- Falls short: the calendar is desktop only. Thunderbird for Android does email, not calendars.
- Pick it if you want one open source app for mail and calendar on your computer.
6. Fossify Calendar: the best Android calendar app
6. Fossify Calendar
An ad-free Android calendar with day, week, month and year views, widgets and event reminders.
Best for: Android users who want a clean, ad-free calendar with widgets.
Official screenshot from github.com/FossifyOrg/Calendar, © the Fossify Calendar project- License
- GPL-3.0-only (GNU General Public License v3.0)
- Stars
- 2.2k (as of Sep 2026)
- Latest release
- 1.11.0 (Sep 2026)
- Language
- Kotlin
- Platforms
- Android
- Self-host
- No server
- API
- None
- MCP
- None found
Source code · Website · Facts checked Sep 2026
Verdict: the calendar app most Android users should start with. It is the community fork of Simple Calendar.
- Good: day, week, month and year views, widgets and reminders, with no ads.
- Falls short: it does not sync by itself. Pair it with DAVx⁵ to reach your own server. There is no iOS app.
- Pick it if you use Android and want a calendar that is simple to read.
7. DAVx⁵: the sync bridge for Android
7. DAVx⁵
An Android app that syncs calendars, contacts and tasks from any CalDAV or CardDAV server.
Best for: Android users who want their phone to sync with a self-hosted calendar.
Note: It has no calendar screen of its own; it syncs events into your phone's calendar app.
Official screenshot from github.com/bitfireAT/davx5-translations, © the DAVx⁵ project- License
- GPL-3.0-only (GNU General Public License v3.0)
- Stars
- 2.9k (as of Sep 2026)
- Latest release
- v4.5.19-ose (Aug 2026)
- Language
- Kotlin
- Platforms
- Android
- Self-host
- No server
- API
- CalDAV, CardDAV, WebDAV
- MCP
- Official
Source code · Website · Docs · Facts checked Sep 2026
Verdict: the piece that makes a self-hosted calendar work on Android.
- Good: it syncs calendars, contacts and tasks from any CalDAV or CardDAV server into Android. Then any calendar app can show them. It is free on F-Droid and paid on Google Play.
- Falls short: it has no calendar screen. You still need Fossify, Etar or another calendar app.
- Pick it if you run your own server and use Android. You almost always need it.
8. GNOME Calendar: the simple Linux calendar
8. GNOME Calendar
The simple calendar app for the GNOME desktop on Linux, with month, week and agenda views.
Best for: Linux desktop users who want a calendar that fits the GNOME desktop.
Official screenshot from the project's README, © the GNOME Calendar project- License
- GPL-3.0-or-later (GNU General Public License v3.0 or later)
- Stars
- 128 (as of Sep 2026)
- Latest release
- 51.0 (Sep 2026)
- Language
- C
- Platforms
- Linux
- Self-host
- No server
- API
- CalDAV
- MCP
- Community
Source code · Website · Facts checked Sep 2026
Verdict: a clean, quiet calendar for the GNOME desktop.
- Good: it reads Google, Nextcloud and Microsoft accounts through GNOME Online Accounts, or any CalDAV server.
- Falls short: Linux only, and fewer features than Thunderbird. No mail, no tasks.
- Pick it if you live on GNOME and want a calendar with no clutter.
9. Etar: a simple Android calendar
9. Etar
A simple Android calendar with month, week, day and agenda views, built on the old AOSP calendar.
Best for: Android users who want the plain, classic Android calendar look.
Official screenshot from github.com/Etar-Group/Etar-Calendar, © the Etar project- License
- GPL-3.0-only AND Apache-2.0 (GNU GPL v3.0, with original AOSP code under Apache License 2.0)
- Stars
- 2.6k (as of Sep 2026)
- Latest release
- v1.0.57 (Jul 2026)
- Language
- Java
- Platforms
- Android
- Self-host
- No server
- API
- None
- MCP
- None found
Source code · Website · Facts checked Sep 2026
Verdict: a steady, no-frills option built on the old Android calendar.
- Good: month, week, day and agenda views, and it has shipped updates for years.
- Falls short: like Fossify, it needs DAVx⁵ to reach a CalDAV server. The design feels older.
- Pick it if you want the classic Android calendar look.
10. Proton Calendar: encrypted, with open apps and a closed server
10. Proton Calendar
An end-to-end encrypted calendar from Proton, on the web and on Android and iOS.
Best for: People who want an encrypted calendar and don't want to run a server.
Note: Only the apps are open source; Proton's servers are not, so you can't self-host it.
Official screenshot from proton.me, © Proton AG- License
- GPL-3.0-only (GNU General Public License v3.0)
- Stars
- 5.6k (as of Sep 2026)
- Latest release
- 5.0.134.1 (Sep 2026)
- Language
- TypeScript
- Platforms
- Web, iOS, Android
- Self-host
- No server
- API
- None
- MCP
- Community
Source code · Website · Docs · Facts checked Sep 2026
Verdict: the easiest private calendar, if you trust Proton to host it.
- Good: end-to-end encryption, with polished apps on the web, Android and iOS. The app code is public under GPL-3.0.
- Falls short: Proton's servers are closed, so you can't self-host it. It has no public API and no CalDAV, so other calendar apps can't sync with it.
- Pick it if you want privacy from Google without running a server.
11. Tuta Calendar: encrypted, with apps everywhere
11. Tuta Calendar
An end-to-end encrypted calendar from Tuta, on the web, desktop, Android and iOS.
Best for: People who want an encrypted calendar with apps on phone and desktop.
Note: Only the apps are open source; Tuta's servers are not, so you can't self-host it.
Official screenshot from tuta.com, © Tutao GmbH- License
- GPL-3.0-only (GNU General Public License v3.0)
- Stars
- 7.9k (as of Sep 2026)
- Latest release
- 360.260922.0 (Sep 2026)
- Language
- TypeScript
- Platforms
- Web, macOS, Windows, Linux, iOS, Android
- Self-host
- No server
- API
- None
- MCP
- None found
Source code · Website · Docs · Facts checked Sep 2026
Verdict: Proton's closest rival, with apps on every platform.
- Good: end-to-end encryption and apps for web, Windows, Mac, Linux, Android and iOS, including F-Droid.
- Falls short: the same deal as Proton. The apps are open source, but the servers are closed, so you can't self-host it.
- Pick it if you want an encrypted calendar with desktop apps too.
12. Cal.diy: open source booking links
12. Cal.diy (Cal.com community edition)
A self-hosted booking page tool: share a link and people pick a free time on your calendar.
Best for: People who want a self-hosted booking page, like Calendly, on their own server.
Note: This is Cal.com's old open-source repo, renamed; it drops team and enterprise features and is meant for personal self-hosting.
Official screenshot from the project's README, © the Cal.diy project- License
- MIT (MIT License)
- Stars
- 48.6k (as of Sep 2026)
- Latest release
- v6.2.0 (Mar 2026)
- Language
- TypeScript
- Platforms
- Web, Server
- Self-host
- Docker, Node.js
- API
- REST, Webhooks
- MCP
- Community
Source code · Website · Docs · Facts checked Sep 2026
Verdict: the open source way to let people book time with you.
Cal.com's open source repo is now called Cal.diy. It is the MIT-licensed community edition, with team and enterprise features taken out.
- Good: booking pages that read your Google, Microsoft or CalDAV calendar and only offer free times.
- Falls short: the project says it is for personal use, not production. For teams, look at hosted Cal.com instead.
- Pick it if you want a personal booking link on your own server.
How to replace Google Calendar with open source
You need three things: a server to hold your events, a way to sync, and an app on each device. Here is the path most self-hosters take.
- Pick a server. Nextcloud Calendar if you want a web calendar and sharing. Radicale or Baïkal if you only need sync. If you don't want to run a server, Proton or Tuta host it for you.
- Put it behind HTTPS. Use your own domain and a reverse proxy, then back up the data folder every night. Our self-hosted productivity guide covers both.
- Export from Google. In Google Calendar, open Settings, then Import & export, then Export. You get a .zip with one .ics file per calendar.
- Import your events. In Nextcloud Calendar, use Import calendar in its settings. For Radicale or Baïkal, add the server to Thunderbird, then use Events and Tasks, then Import.
- Connect your phone. On Android, install DAVx⁵, then view events in Fossify Calendar or Etar. On an iPhone, add a CalDAV account in Settings, under Calendar accounts.
- Connect your computer. Thunderbird works on Windows, Mac and Linux. GNOME Calendar and the Mac's own Calendar app can add CalDAV accounts too.
- Plan your days. Keep a calendar for meetings, and a planner for the work between them.
One honest note on step 7. Open Sunsama connects to Google, Outlook and iCloud today. Its iCloud form takes a custom CalDAV URL, but those three are the calendars we support.
Open source licenses in plain words
Every app here shows its license on its card. Here is what those licenses mean for you.
| License |
What it means for you |
| MIT (Cal.diy) |
Use, change and share the code for anything. |
| GPL-3.0 (Radicale, Baïkal, Fossify, Etar, DAVx⁵, GNOME Calendar, Proton, Tuta) |
Use and change it freely. If you share a changed version, share your code too. |
| AGPL-3.0 (Nextcloud Calendar) |
Like GPL, and it also covers apps you run for others over the web. |
| MPL-2.0 (Thunderbird) |
Share changes to its own files; the rest of your project can use any license. |
| Non-commercial (Open Sunsama) |
Read it, run it and self-host it for personal use. Companies need a commercial license. |
For personal use and self-hosting, all of these give you full control. Open Sunsama's license is not an OSI license like MIT or GPL, so check it if you are a company.
Which calendars work with AI agents?
An AI agent talks to apps over MCP. Here is where each calendar stands.
- Open Sunsama: a hosted MCP server with OAuth sign-in. Claude, ChatGPT or Cursor can read your meetings and plan time blocks around them. Calendar events are read-only over MCP. See the MCP setup guide.
- Nextcloud Calendar: Nextcloud's Context Agent app has an MCP server with calendar tools. It needs the AppAPI setup.
- Radicale and Baïkal: community CalDAV MCP servers work with any CalDAV server.
- DAVx⁵: its makers publish davmcp, an early MCP server for CalDAV events. Its own README says it is not ready for real use.
- Proton and Tuta: no public calendar API, so only small community projects exist, or none.
Which one should you pick?
- For most people: Open Sunsama. Keep your calendar where it is, and plan your tasks right next to your meetings.
- If you want Google Calendar on your own server, pick Nextcloud Calendar.
- If you want the smallest possible server, pick Radicale, or Baïkal on a PHP host.
- If you want a desktop app, pick Thunderbird. On GNOME, pick GNOME Calendar.
- If you use Android, pick DAVx⁵ plus Fossify Calendar.
- If you want privacy but no server, pick Proton Calendar or Tuta Calendar.
- If you want booking links, pick Cal.diy.
Planning your whole stack? See our picks for open source planner apps, to-do list apps, kanban boards and focus apps, or the full list of open source productivity apps.
Frequently asked questions
What is the best open source calendar app?
Open Sunsama is our pick to plan your day on top of your calendar, with time blocks and any AI agent. To host the calendar itself, Nextcloud Calendar is the fullest option and Radicale is the lightest. For apps, use Thunderbird on desktop and Fossify Calendar on Android.
What is the best open source Google Calendar alternative?
Nextcloud Calendar is the closest: a web calendar with sharing, invites and booking on your own server. If you only need sync, run Radicale or Baïkal and use the calendar apps you already have. Proton Calendar is open source on the app side, but you can't self-host it.
How do I self-host a calendar?
Run a CalDAV server such as Radicale, Baïkal or Nextcloud, put it behind HTTPS, and add it as a CalDAV account on each device. Android needs DAVx⁵ to sync. iPhones, Macs and Thunderbird can add CalDAV accounts on their own.
What is CalDAV?
CalDAV is the open standard that calendar apps use to sync events with a server. Any CalDAV app can talk to any CalDAV server, so you can mix and match. CardDAV does the same job for contacts.
Can I use a self-hosted calendar on my iPhone?
Yes. The iPhone's own Calendar app can add a CalDAV account in Settings, under Calendar accounts. Enter your server address, user name and password, and your events sync both ways.
Is Proton Calendar open source?
The apps are. Proton publishes the web, Android and iOS code under GPL-3.0. The servers are closed, so you can't self-host Proton Calendar. Tuta Calendar works the same way.
Is Open Sunsama open source?
The code is public on GitHub. It uses a non-commercial license: you can read it, run it and self-host it for personal use. Companies need a commercial license. It is not an OSI license like MIT or GPL.
Is there an open source Calendly alternative?
Yes. Cal.diy is the MIT-licensed community edition of Cal.com, built for personal self-hosting. Nextcloud Calendar also has appointment booking built in.