Add Full Two‑Way Sync Support for Outlook and Google Calendar

Is your feature request related to a problem? Please describe.

Hello Nextcloud Team,

First, thank you for building such an excellent and privacy‑respecting calendar application. Nextcloud Calendar embodies the core philosophy of the platform: users should fully own and control their data without relying on commercial cloud providers. I use Nextcloud together with my NAS and Docker environment, and Calendar has been one of the most reliable components in my self‑hosted setup.

I would like to propose a feature that could significantly enhance the usability and adoption of Nextcloud Calendar:
full two‑way synchronization with Outlook (Exchange/Office 365) and Google Calendar.

Why this feature matters
Real‑world calendar ecosystems are mixed.
Many users manage work schedules through Outlook/Exchange and personal schedules through Google Calendar. Nextcloud Calendar becomes far more powerful if it can bridge these environments seamlessly.

It aligns perfectly with Nextcloud’s philosophy.
Providing interoperability does not compromise privacy. Instead, it empowers users to centralize their calendars under their own control while still interacting with external systems when needed.

Technically feasible.
CalDAV ↔ Exchange ↔ Google Calendar API synchronization has been implemented in various open‑source tools. A native, stable implementation within Nextcloud would greatly improve reliability and user experience.

Huge impact on adoption.
With this feature, Nextcloud Calendar could become the definitive open‑source alternative to commercial calendar platforms, especially for organizations transitioning away from proprietary ecosystems.

Summary
Nextcloud Calendar is already an outstanding app. Adding full Outlook/Google two‑way sync would elevate it into a complete, universal scheduling solution. I hope this suggestion is helpful for future development, and I deeply appreciate the work the team continues to put into this project.

Thank you for your time and dedication.

Describe the solution you'd like

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Describe alternatives you've considered

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Additional context

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Start by reviewing the Calendar app's existing CalDAV integration points and how external calendar services could connect to Outlook/Exchange/Office 365 and Google Calendar. Done would require a defined, stable full two-way synchronization design, but this issue provides no files, tests, or concrete implementation scope.