What is CalDAV and how does it work for syncing calendars?

CalDAV is an internet protocol based on WebDAV used for synchronizing and managing calendar data. It enables users to synchronize calendars across devices with a server. CalDAV is often used in email and calendar services like Apple Calendar, Thunderbird, or Nextcloud.

What is CalDAV?

The acronym CalDAV stands for “Calendar Distributed Authoring and Versioning.” CalDAV is a network protocol that was first proposed to the IETF (Internet Engineering Task Force) in 2003 and officially specified in 2007 in RFC 4791. It facilitates calendar data exchange over WebDAV/HTTP, enabling you to access and sync your calendar across any device, from anywhere. Any changes you make, whether adding new appointments or modifying existing entries, are automatically reflected on all synchronized devices, including your PC, smartphone, or tablet.

How does CalDAV work?

CalDAV enables the management and editing of appointments, events, and time blocks across platforms on all devices. The protocol handles and distributes calendar events using the HTTP transfer protocol, which is commonly used on the web. All events published by CalDAV are formatted in the iCalendar format, allowing any standard browser to display a downloaded appointment or event in iCalendar format.

A WebDAV environment serves as the basis for management and synchronization, which, thanks to CalDAV, is extended with access control and iCalendar evaluation functions. You can also set up a write lock for other users, transmit available time slots, and merge parallel changes to an entry. This way, you can not only synchronize your calendar on multiple personal devices but also share it with other users. If you want to synchronize your contact data as well, there is also the CardDAV protocol.

To use CalDAV, you need to implement the network protocol into your calendar applications by downloading and installing a CalDAV client. Through this program, you then establish a connection with a CalDAV server, which regulates the calendar data exchange as a central instance.

What is a CalDAV client and what is a CalDAV server?

A CalDAV client refers to a computer program that runs on a smartphone, tablet, or PC, communicating with a central computer—called a server—via the CalDAV protocol to send and receive calendar data. Often, the device itself is also referred to as a client. The client software must support not only the network protocol but also your device. For example, there are different clients for Windows, Android, or Apple devices.

A CalDAV server is the counterpart to the CalDAV client: As the point of contact for all requesting clients, it forms the basis for data exchange and thus for the entire calendar synchronization. This means you need a CalDAV server where your data resides, and an appropriate client on your device to retrieve or display the data from the server.

An overview of the most important CalDAV servers and clients

Suitable clients for synchronizing calendar data via CalDAV include:

• CalendarSync for Android
• AgenDAV
• Atmail (from version 6.0)
• Outlook CalDav Synchronizer (free Outlook plug-in)
• iPhone

For a server, a basic model is sufficient to use CalDAV, which many providers offer for free.

• Calendar and Contacts Server (macOS and Linux)
• Atmail (from version 6.0)
• Baïkal Lightweight CalDAV+CardDAV server
• Bedework
• Google Calendar (limited support, read-only; better to use the Google CalDAV API)
• ownCloud
• Nextcloud

How to implement CalDAV

If you want to set up your own CalDAV server, you definitely need a static IP address. Once the server is up, download the CalDAV client of your choice, install it, and then create an account. Connect the profile to the server by providing the relevant details, such as the network address you previously set. Next comes the fine-tuning, where you set the desired options—like who you want to share the calendar with.

Examples for using the CalDAV protocol

In organizations with mixed IT environments or individual calendar infrastructures, CalDAV provides the ability to manage appointments centrally and integrate them flexibly into existing workflows. Below, we show how CalDAV can be integrated into popular environments like Nextcloud, Microsoft 365, and Google Workspace.

Managing calendars with Nextcloud and CalDAV for seamless synchronization

Nextcloud natively supports CalDAV through its integrated calendar app. Users can manage their calendar data directly in the web interface and sync with external clients via CalDAV. You can copy the CalDAV access URL from your calendar app user account. It typically looks like this: https://cloud.example.com/remote.php/dav/calendars/username/. With a compatible CalDAV client, calendars can easily be subscribed to or actively edited. Nextcloud also allows permission settings for shared calendars, making it ideal for teamwork.

Integrating CalDAV with Outlook using CalDav Synchronizer

Microsoft 365 does not offer native CalDAV support. However, it’s possible to integrate CalDAV services through third-party tools like CalDav Synchronizer for Outlook. These tools act as a bridge between Microsoft’s Exchange services and the open CalDAV standard. This approach is particularly relevant for companies with hybrid IT infrastructures or when combining calendars from Nextcloud or Apple iCloud with Outlook. However, functional and permission limitations should be expected.

The following short guide explains the procedure with CalDav Synchronizer in Outlook:

1. Download the plugin and install it. Close Outlook before installation.
2. Click on the “CalDav Synchronizer” tab in Outlook and create a new CalDAV profile. Select “Generic CalDAV” as the profile type.
3. Select the Outlook folder to be synchronized and enter the CalDAV URL from your CalDAV provider.
4. Save the profile to complete the setup.

Using CalDAV with Google Workspace

The CalDAV URL for accessing Google Workspace or Google Calendar is https://apidata.googleusercontent.com/caldav/v2/your_email_address/events. To connect, you must sign in with your Google account—usually through an app password or OAuth, as simple passwords are often no longer accepted. You can then connect your CalDAV client with this URL to synchronize your appointments in read-only mode.

Note that Google CalDAV is primarily optimized for retrieving calendar data. Therefore, full editing of appointments is often limited. For more extensive calendar functions, it is recommended to use the “Google Calendar” API, which allows for advanced management and synchronization.

Using CalDAV with IONOS

IONOS also offers you an easy way to import calendar data and synchronize across platforms using CalDAV. This allows you to, for example, migrate your calendar from Webmail to Microsoft 365 and conveniently view and edit your appointments on various devices—without data loss or duplication. Setup is done via the CalDAV URL, which you can find in your IONOS customer account, and by connecting with a CalDAV-capable calendar client.