CalIn — ICS Calendar to Exchange Sync#

Sync busy time from any ICS/iCal calendar to Microsoft Exchange as "Blocked" slots. Works with Google Calendar, iCloud, Outlook.com, Nextcloud, Fastmail, Proton Calendar, or any CalDAV server that publishes an ICS feed.

When to Use This Skill#

• User asks to sync calendars, block time, or mirror busy slots
• User wants personal calendar events to block time on their work calendar
• User asks to check sync status or clear synced events
• Scheduled background sync via Heartbeat (recommended: every 5-30 minutes)

What It Does#

• Fetches events (including recurring) from any HTTPS ICS feed
• Creates/updates/deletes corresponding "Blocked" events on Exchange via Microsoft Graph API
• Tracks synced events with a local mapping file — only touches what changed
• Privacy-preserving: only syncs time slots, never event titles, descriptions, or attendees

Setup#

1. Install dependencies#

Requires Python 3.12+.

bash

cd ~/.openclaw/skills/calin
bash install.sh

This creates a Python venv and installs icalendar, msal, recurring-ical-events, and requests.

2. Configure credentials#

Add to ~/.calintegration/.env (created by installer with chmod 600):

text

CALINT_ICS_URL=<your-ics-feed-url>
CALINT_MS_CLIENT_ID=<azure-app-client-id>
CALINT_MS_TENANT_ID=<azure-app-tenant-id>
# Optional: sync to a specific Exchange calendar (defaults to primary)
# CALINT_MS_CALENDAR_ID=<calendar-id>

Where to find your ICS URL:

ProviderLocationGoogle CalendarSettings > your calendar > Integrate > "Secret address in iCal format"iCloudCalendar app > right-click calendar > Share > Public Calendar > copy URLOutlook.comSettings > Calendar > Shared calendars > Publish > ICS linkNextcloudCalendar > three-dot menu > Copy subscription linkFastmailSettings > Calendars > Share/export > ICS URL

Azure AD app (free, no client secret needed):

1. Go to https://portal.azure.com/#view/Microsoft_AAD_RegisteredApps/ApplicationsListBlade
2. New registration > Name: "CalIn" > Redirect URI: Public client > http://localhost:8400
3. API permissions > Add > Microsoft Graph > Delegated > Calendars.ReadWrite
4. Copy Application (client) ID and Directory (tenant) ID

3. Authenticate Microsoft#

bash

cd ~/.openclaw/skills/calin
venv/bin/python main.py setup

This opens a browser for one-time Microsoft login. Tokens are cached locally.

Commands#

Run these from the skill directory:

CommandWhat it doesvenv/bin/python main.py syncRun one sync cyclevenv/bin/python main.py statusShow last sync time and event countvenv/bin/python main.py clearRemove all synced events from Exchangevenv/bin/python main.py setupRe-run authentication setup

Automated Sync (macOS)#

The installer generates a launchd plist for automatic sync every 5 minutes:

bash

cd ~/.openclaw/skills/calin
bash install.sh
cp com.calintegration.sync.plist ~/Library/LaunchAgents/
launchctl load ~/Library/LaunchAgents/com.calintegration.sync.plist

How It Works#

1. Fetches the ICS feed and expands all recurring events within a 14-day window
2. Compares against the local event map (previous sync state)
3. For each event:
   • New: Creates a "Blocked" event on Exchange
   • Changed (time moved): Updates the Exchange event
   • Deleted from source: Deletes the Exchange event
4. Saves updated event map and last sync timestamp

Change detection uses a hash of start time + end time + all-day flag. Only changed events trigger API calls.

Security and Privacy#

• Credentials stored in ~/.calintegration/.env with chmod 600 permissions
• Tokens stored in ~/.calintegration/ms_token.json with chmod 600 permissions
• All files in ~/.calintegration/ directory with chmod 700
• No event details synced — only time slots are transmitted. Event titles, descriptions, attendees, and locations are never sent to Exchange
• HTTPS only — ICS URLs must use HTTPS; Graph API calls use TLS with certificate verification
• No client secret — uses PKCE (Proof Key for Code Exchange) for Microsoft auth
• ICS URL validated — rejects non-HTTPS schemes to prevent SSRF
• API path IDs validated — prevents injection in Graph API endpoint construction
• All HTTP requests have 30-second timeouts to prevent hanging

External endpoints called#

EndpointPurposeAuthYour ICS URL (HTTPS)Read calendar eventsURL contains secret tokenlogin.microsoftonline.comMicrosoft OAuth token exchangePKCE flowgraph.microsoft.com/v1.0Create/update/delete Exchange eventsBearer token

Data flow#

text

ICS Feed (read-only) --> [CalIn on your machine] --> Microsoft Graph API (write)
                              |
                     ~/.calintegration/ (local state)

No data is sent to any third party. All processing happens locally.