How to Sync Nextcloud Calendar with Microsoft 365 / Outlook

Your calendar lives in Nextcloud, but your colleagues, your employer or your customers book meetings in Microsoft 365. Sooner or later you want both in one place: your Nextcloud appointments visible in Outlook, and Outlook meetings in Nextcloud. Here's why that doesn't work out of the box, which workarounds people try, and how to get a real two-way sync.

Why Nextcloud and Microsoft 365 don't talk to each other

Nextcloud stores calendars on a CalDAV server (/remote.php/dav), the open standard (RFC 4791) that Thunderbird, Apple Calendar or DAVx⁵ speak. Microsoft 365 and Outlook.com don't support CalDAV at all. Their calendars are reachable through Exchange and the Microsoft Graph API.

So there's no setting in Outlook where you could enter your Nextcloud address, and Nextcloud can't log in to Microsoft 365 either. The two systems need something in between.

Workarounds and their limits

Subscribe to an ICS link

Nextcloud can share a calendar as a public link, and Outlook can add a calendar "from the internet". It looks like a sync, but:

• the subscribed calendar is read-only in Outlook, so you can't create or move events there,
• Microsoft refreshes it on its own schedule, and you can't force an update. New appointments can take hours to show up,
• it only works in one direction. Subscribing the other way round too gives you two read-only copies, not a sync.

Outlook CalDAV Synchronizer

The open-source add-in syncs CalDAV calendars into classic Outlook for Windows. The new Outlook doesn't run COM add-ins, and Microsoft is gradually replacing classic Outlook with it. In addition, the add-in only syncs into the local Outlook profile, so Outlook on the web, the mobile app and Teams never see the events. More on that in Outlook CalDAV Synchronizer alternative.

Keeping two calendars by hand

Copying meetings manually or forwarding invitations works until the first change is missed. Then you have double bookings.

The solution: server-to-server sync with CalDAVconnect

CalDAVconnect connects your Nextcloud via CalDAV and your Microsoft account via the Microsoft Graph API, and syncs the two in the cloud. Nothing needs to be installed on your computer or phone.

• Two-way: create, change or delete an event in Nextcloud or in Outlook, and the change appears on the other side. One-way sync is available too, if one side should stay the single source of truth.
• Within seconds: Microsoft notifies CalDAVconnect of changes via webhooks, so they reach Nextcloud within 1–2 seconds. In the other direction CalDAVconnect checks your Nextcloud calendars every 15 seconds with a single lightweight request; on average a change shows up in Outlook after 7–8 seconds.
• Visible everywhere: events land in the Microsoft 365 cloud calendar, so they show up in the new and classic Outlook, Outlook on the web, Outlook mobile and Teams.
• All your Nextcloud calendars: one connection covers every calendar on your Nextcloud. You pair each Nextcloud calendar with an Outlook calendar, or let CalDAVconnect create a new one.
• Work and personal accounts: Microsoft 365 work and school accounts are supported as well as personal Outlook.com accounts.

Only "busy" for the company calendar

A common case: your private Nextcloud calendar should block time in your company's Microsoft 365 calendar, without colleagues seeing what the appointment is about. For that, each calendar pair has a privacy mode. It replaces the title with "Busy" or a label of your choice and removes description, location and attendees. Strict even transfers only the time block, without the organizer.

Setting it up

Setup takes about five minutes:

1. Create a CalDAVconnect account. You get the full version free for 14 days, no credit card required.
2. Add a connection, choose Nextcloud as the server type and enter your Nextcloud address (e.g. https://cloud.example.com). The CalDAV path is added for you.
3. Sign in with an app password, which you create in Nextcloud under Settings → Security → Devices & sessions. If your Nextcloud uses two-factor authentication or single sign-on, an app password is required; see Troubleshooting.
4. Connect your Microsoft account. If your organization restricts third-party apps, an admin may need to approve access once.
5. Pair your calendars. The first sync starts automatically.

Details for every step are in the documentation: Connect your CalDAV server and Connect Google or Microsoft.

Privacy and hosting

CalDAVconnect runs on Hetzner in Germany. CalDAVconnect itself processes and stores calendar data only in the EU, and credentials and event data are encrypted at rest. Events you sync to Microsoft 365 are of course stored at Microsoft as well. A data processing agreement applies to every plan. IT teams find details on scopes, encryption and deletion on Security & data protection.

Using Google Calendar too?

The same works for Google: How to sync Nextcloud Calendar with Google Calendar. For CalDAV servers other than Nextcloud, see Office 365 and CalDAV. Comparing services? CalDAVconnect vs Kalender Sync puts both side by side.

Conclusion

Microsoft 365 won't speak CalDAV, and ICS subscriptions or the Outlook CalDAV Synchronizer only cover part of the problem. CalDAVconnect keeps your Nextcloud calendar and Microsoft 365 in sync in both directions, within seconds and on every device.