Anyone who uses Google Workspace for work and Apple iCloud for personal use—or who juggles multiple Outlook accounts as a freelancer—knows the problem: double bookings, manual synchronization, and the uneasy feeling when appointment details end up in other systems. The Kalender-Sync solves this by synchronizing only free/busy availability between calendars. No titles, no attendees, no notes leave the respective account. The target calendar only displays "Blocked"—the time slot is reserved, the content remains private.
Google Calendar, Microsoft Outlook, Apple iCloud, Infomaniak KSuite, and any iCal feeds are supported. The entire infrastructure is hosted by Hetzner in Nuremberg, and a data processing agreement is available.
Kalender-Sync is aimed at freelancers, self-employed individuals, small teams, and families in the DACH region who use multiple calendars and want to avoid double bookings without sharing sensitive data with US services. Currently free to use; pricing will be introduced in the coming weeks.
Paul Koch is an AI Product Engineer in Munich. He built Kalender-Sync out of his own need, after having to juggle multiple calendar accounts for different clients and not finding a GDPR-compliant solution. 
