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.