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New: Connect Claude & ChatGPT to your calendars – the AI assistant is here

Kalender Sync
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Integrations

Sync the calendars you already use

Google, Microsoft Outlook (M365 + Exchange Online), Apple iCloud, GMX, WEB.DE, mailbox.org and Infomaniak KSuite — all bidirectional — plus any iCal feed (read-only), all GDPR-compliant, all hosted in a German data centre. Five more providers already on the roadmap.

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8 live · 5 coming up

Providers at a glance

Eight providers, one sync layer

Eight are live today. Five are in "Coming up" on the roadmap — click the card to leave your vote.

Google Calendar

Available

Google Workspace and personal Gmail calendars via OAuth.

  • OAuth 2.0
  • Real-time webhook
  • Workspace & Gmail
Sync Apple & Google →

Microsoft Outlook

Available

Microsoft 365, Outlook.com and Exchange Online — no add-in needed.

  • OAuth 2.0
  • Microsoft Graph
  • M365 + Exchange
Sync Outlook & Google →

Apple iCloud

Available

iCloud calendars via CalDAV and an app-specific password.

  • CalDAV
  • App-specific password
  • iPhone, iPad, Mac
Sync Apple & Google →
K

Infomaniak KSuite

Available

Swiss calendar provider with CalDAV. EU data stays in the EU.

  • CalDAV
  • EU hosting
  • Swiss provider
See details →

GMX

Available

Germany's classic personal mail provider — GMX calendar via CalDAV, from the Basic plan.

  • CalDAV
  • App-specific password
  • German provider
See details →

WEB.DE

Available

Connect your WEB.DE calendar via CalDAV — from the Basic plan.

  • CalDAV
  • App-specific password
  • German provider
See details →

mailbox.org

Available

German business mail provider from Berlin — calendar via CalDAV, from the Basic plan.

  • CalDAV
  • German provider
  • Business mail
See details →

iCal feeds

Available

Subscribe to any external calendar via iCal URL — read-only.

  • URL-based
  • Read-only
  • Outlook/Calendly/etc.
See details →

Fastmail

Coming up · 9 votes

CalDAV-based Fastmail calendar connection — no mail access.

  • CalDAV
  • App-specific password
  • EU/AU hosting
Upvote on roadmap →

Proton Calendar

Coming up · 7 votes

Proton Calendar — on our wishlist, blocked on Proton offering third-party calendar access. Read-only ICS import under review.

  • Pending Proton API availability
  • EU hosting
  • Privacy-focused
Upvote on roadmap →
Y!

Yahoo Calendar

Coming up · 5 votes

The long tail of personal calendars — Yahoo via CalDAV.

  • CalDAV
  • App-specific password
  • Personal calendar
Upvote on roadmap →

Nextcloud

Coming up · 0 votes

Self-hosted calendars via CalDAV — the open-source cloud.

  • CalDAV
  • Self-hosted
  • Open source
Upvote on roadmap →

Posteo

Coming up · 0 votes

Germany's green privacy mail provider — calendar via CalDAV.

  • CalDAV
  • German provider
  • Privacy-first
Upvote on roadmap →

Setup per provider

How the connection actually works

Per provider: which auth method, which scopes, which sub-services, common combinations. So you know what you're getting before you click.

Google Calendar

Google Calendar is the most common source calendar for our users. We connect Workspace accounts (Google Workspace / G Suite) as well as personal Gmail accounts — both via OAuth 2.0, no admin rights needed on your end. Connection takes ~30 seconds; from then on, sync runs over Google's push-notifications webhook — typical latency seconds to a few minutes, backed by a 4-hour reconciliation job as safety net.

Setup flow

  1. 1 On sign-in with Google you see the familiar Google consent screen — no detours.
  2. 2 We receive an OAuth token, never your password. Tokens are encrypted at rest with ChaCha20-Poly1305.
  3. 3 Want to connect two or more Google accounts? Works — each gets its own auth session.

Worth knowing: No contact, mail or Drive permissions — we don't request them. That's also Google's own guidance: scope minimisation is a precondition for verified-publisher status.

Microsoft Outlook

"Microsoft Outlook" is shorthand for: Microsoft 365 Business, Outlook.com (personal) and Exchange Online — hybrid tenants work through the same multi-tenant endpoint. All via the Microsoft Graph API — one endpoint, OAuth 2.0, no Outlook add-ins, no COM wrappers. Tenants with strict consent policy get a tenant-admin-consent link (see /en/it-admin/).

Setup flow

  1. 1 Microsoft sign-in opens the standard consent screen for your tenant.
  2. 2 You choose which of the connected calendars (default + sub-calendars) serves as source.
  3. 3 Tenants with strict publisher policy: one click on the admin-consent link at /en/it-admin/, your Global Admin grants once.

Worth knowing: On-premises Exchange (self-hosted, without Microsoft 365): not directly supported — but via an iCal feed export you can wire up a read-only connection. Microsoft Publisher Verification is in progress (see roadmap).

Apple iCloud

Apple iCloud runs on CalDAV — there is no official OAuth API. Connection works via an app-specific password that you create at your Apple ID under "Sign-In & Security → App-Specific Passwords". iCloud requires 2FA for app-specific passwords, so you need to enable two-factor on your Apple ID once.

Setup flow

  1. 1 At appleid.apple.com → create an app password (label "Kalender Sync").
  2. 2 In our app: enter your Apple ID email + the generated password — we check CalDAV endpoint reachability immediately.
  3. 3 Apple occasionally redirects to shard servers (caldav.icloud.com → p[NN]-caldav.icloud.com). We follow automatically.

Worth knowing: If you later revoke the app password, we detect it at the next sync cycle, pause affected sharings, and email you. Re-activation: create a new app password, enter it in the app — existing sharings pick up where they left off.

K Infomaniak KSuite

Infomaniak KSuite is the Swiss suite alternative to Workspace/M365 — CH-hosted, privacy-focused. Calendar connection runs over the same CalDAV mechanism as Apple, with an app-specific password from your KSuite account.

Setup flow

  1. 1 In KSuite → Security → create an application-specific password.
  2. 2 In the Kalender Sync app: enter KSuite email + password.
  3. 3 We use Infomaniak's official CalDAV endpoint — no unofficial workarounds.

Worth knowing: KSuite data stays in Switzerland (Infomaniak data centre). Our own server logs for the connection stay in Germany (Hetzner Falkenstein). Relevant for compliance reviews — details at /en/it-admin/.

GMX

GMX is one of Germany's most-used personal email providers — and millions keep their calendar there. The connection runs over CalDAV with an app-specific password, just like Apple iCloud. Your private GMX calendar shows up next to your work calendar in a controlled way: busy-only by default, title + time or full details if you opt in.

Setup flow

  1. 1 On account.gmx.net under "Login & Security", create an app-specific password (your regular GMX password will not work).
  2. 2 In the Kalender Sync app: choose GMX, enter your GMX address + the app-specific password, hit "Test & connect".
  3. 3 Changes are fetched every 5 minutes. GMX is included from the Basic plan.

Worth knowing: German provider meets German sync tool: GMX calendar data and our processing stay fully under GDPR — no US intermediary. The app-specific password can be revoked at GMX at any time, which ends access immediately.

WEB.DE

WEB.DE is, alongside GMX, one of Germany's biggest personal mail providers — for many households, the family calendar lives here. It connects via CalDAV with an app-specific password; after that, syncing with Google, Outlook or iCloud runs automatically, with the visibility level selectable per sync.

Setup flow

  1. 1 In your WEB.DE settings under "Login & Security", create an app-specific password (your regular password will not work).
  2. 2 In the Kalender Sync app: choose WEB.DE, enter your WEB.DE address + the app-specific password, hit "Test & connect".
  3. 3 Changes are fetched every 5 minutes. WEB.DE is included from the Basic plan.

Worth knowing: As with GMX: data and processing stay fully under GDPR. The app-specific password can be deleted in your WEB.DE settings at any time — access ends immediately.

mailbox.org

mailbox.org is the Berlin-based business mail provider for people who want their communication to stay entirely in Germany. Its calendar connects via CalDAV and then runs like any other calendar: two-way sync with Google, Outlook, iCloud and more, visibility level selectable per sync.

Setup flow

  1. 1 Create an app password in your mailbox.org settings (your regular password will not work), then choose mailbox.org in the Kalender Sync app and connect with address + app password.
  2. 2 Pick the calendars and set sync direction + visibility level.
  3. 3 mailbox.org is included from the Basic plan.

Worth knowing: German provider meets German sync tool: mailbox.org hosts in Berlin, Kalender Sync at Hetzner in Nuremberg — data never leaves Germany. The simplest combination on the market for compliance reviews.

iCal feeds

For anything that doesn't speak OAuth or CalDAV: iCal feeds. Any calendar with a public or private ICS URL can serve as source — Calendly, Outlook Published Calendar, Notion, TickTick, university portals, membership systems. Read-only into Kalender Sync, but universal.

Setup flow

  1. 1 In the source system, copy the iCal subscription URL (usually under "Share calendar → private link").
  2. 2 Paste it into our app — we validate that the URL is a valid ICS feed.
  3. 3 Refresh interval: every 5 minutes. Recurring events are expanded correctly.

Worth knowing: Security note: treat private iCal URLs like passwords — anyone with the URL has read access. The feed URL itself is stored in our database; only higher-risk secrets (OAuth tokens, CalDAV passwords) are encrypted at rest with ChaCha20-Poly1305.

Compatibility matrix

Every provider with every other

Bidirectional for the OAuth providers (Google, Outlook). Read-only when the source is an iCal feed.

Source ↓ / Target → GoogleOutlookiCloudKSuiteiCal
Google —✓✓✓✕
Outlook ✓—✓✓✕
iCloud ✓✓—✓✕
KSuite ✓✓✓—✕
iCal ↪↪↪↪—
✓ bidirectional  ·  ↪ read-only (iCal feed as source)  ·  ✕ not possible (iCal feed is not writable)

In comparison

What the non-EU alternatives don't ship

OneCal and CalendarBridge are technically solid, but based outside the EU and awkward for EU compliance reviews. The rows that come up in every sales conversation:

Criterion Kalender Sync OneCal CalendarBridge
GDPR-compliant (Art. 28 DPA) ✓ DPA (non-EU vendor) Partial
EU hosting ✓ — —
Free/Busy without detail sync ✓ ✓ ✓
Real-time webhook (Google + MS) ✓ ✓ —
CalDAV providers (iCloud, KSuite, GMX, WEB.DE) ✓ — —
8 provider types (incl. GMX & WEB.DE) ✓ — —
Per-vendor deep dive: /en/compare/ →

For your IT

Compliance review before rollout?

OAuth scopes per provider, sub-processors with location, DPA under GDPR Art. 28, Microsoft tenant-admin-consent link for strict tenants — all on one page, downloadable as PDF.

IT overview

Hosting

🇩🇪 DE

DPA

Art. 28

Encryption

ChaCha20

Frequently asked

What IT, legal and power users ask

Can I also connect my calendars to an AI assistant?

Yes. Via our MCP server you connect Claude or ChatGPT to all granted calendars – detail level selectable per calendar, revocable any time. All details at kalender-sync.de/en/mcp.

Do I need admin access to connect calendars?

No. Google and Microsoft connect via OAuth — you only need your own account. For iCloud, KSuite, GMX, WEB.DE and mailbox.org, an app-specific password (which you create yourself in the provider settings) is enough. Exception: Microsoft tenants with strict consent policy need a one-time tenant-admin-consent — link at /en/it-admin/.

What permissions does Kalender Sync request?

Calendar and basic profile only. Google: calendar + calendar.events (read+write) plus OpenID Connect scopes (openid, email, profile). Microsoft: Calendars.ReadWrite + User.Read + offline_access plus the OpenID Connect scopes. No contacts, no email, no files. Scope minimisation is required for our Google verification.

Can I connect multiple accounts of the same provider?

Yes. You can connect e.g. two Google accounts and one Microsoft account at the same time. Each account is authenticated separately and has its own sharings. Common case: personal Gmail + Workspace + Outlook.com.

Does it work with personal Gmail the same way as Google Workspace?

Yes, both. Personal Gmail: individual OAuth consent. Google Workspace: if your admin has approved external OAuth apps in general, it works identically — otherwise a single Workspace admin consent is enough.

Do you support self-hosted Exchange (on-premises)?

Not directly — we use the Microsoft Graph API, which targets Microsoft 365 / Exchange Online. Workaround for on-prem Exchange: export your Outlook Published Calendar as an iCal feed and subscribe to it (read-only). True bidirectional support is on the roadmap, no fixed date.

How do I connect Apple iCloud if I have 2FA enabled?

Apple requires app-specific passwords exactly when 2FA is active — which is the default anyway. At your Apple ID → Sign-In & Security → App-Specific Passwords → create a new password with the label "Kalender Sync". Copy the password once, paste it into the app, done.

Are my credentials stored?

For Google and Microsoft: no — we use OAuth tokens, not your password. For iCloud, KSuite, GMX, WEB.DE and mailbox.org: app-specific passwords are stored authenticated-encrypted with ChaCha20-Poly1305 and are server-side readable only. All connections over TLS 1.3.

How fast is the sync?

Google and Microsoft: push-notification webhooks — typical latency seconds to a few minutes, backed by a 4-hour reconciliation job as safety net. Apple iCloud, KSuite, GMX, WEB.DE, mailbox.org (CalDAV) and iCal feeds: polled every 5 minutes. On provider throttling we fall back to longer intervals without pausing sharings.

What happens if a provider has a 1-hour outage?

We queue changes and retry with exponential backoff (3 attempts, 30s initial delay). Paused sharings reactivate automatically as soon as the provider responds again — no duplicated or lost events.

My provider isn't here. Can I suggest it?

Yes — at /en/roadmap/ you'll find the public voting board. Fastmail and Yahoo are already in "Coming up"; Proton Calendar is parked until Proton offers third-party access, and Nextcloud and Posteo are on the integrations wishlist. What you upvote there we see directly — voting is anonymous, but it weights the roadmap.

Ready to connect your calendars?

Try free for 14 days

Currently supported providers

Google Microsoft Apple iCloud KKSuite GMX WEB.DE mailbox.org iCal-Feeds

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