I built a GDPR-compliant calendar sync for the EU market — Closed Beta is live
Hey IH 👋
Quick update on Kalender-Sync — I posted about it here a few months ago when it was still an idea. It's now a working product in Closed Beta with real users.
The problem: If you use Google Calendar for personal stuff and Outlook for work (or vice versa), your calendars don't talk to each other. You end up with double bookings, manual copy-pasting, or sharing way too much with your employer or 3rd parties.
What Kalender-Sync does:
- Syncs Google ↔ Microsoft/Outlook calendars automatically
- You choose what to share: just Free/Busy, title + time, or full details
- Self-Sync (merge your own calendars) or share with colleagues/clients via invite link
- No calendar content stored — only technical mappings
- GDPR-compliant, hosted on Hetzner in Germany
Where I'm at:
- 6 beta users actively testing
- ~20,000 events synced (and one fun bug where bidirectional sync created an infinite loop — fixed now 😅)
- Built with: Node.js, Fastify, PostgreSQL, React, deployed on a single Hetzner VPS
What I learned so far:
- Google's Calendar API rate limits return 403 (not 429 like you'd expect). Their own retry library doesn't handle it. Had to build custom exponential backoff.
- Bidirectional sync is deceptively hard. Calendar A → B and B → A sounds simple until every synced event triggers a webhook that syncs it back. Loop detection was a must.
- Users care way more about privacy than I expected.
Biggest challenge right now: Getting more beta users. I have a small list (~30 newsletter subscribers) from organic SEO, but converting them to active testers is slow. Google OAuth test mode means I have to manually add every user — big friction.
Would love to hear:
- How did you get your first 10-20 beta users for a niche B2C/prosumer tool?
- Anyone here syncing calendars between providers? What's your current workaround?
DM me if you want a beta invite
The 'users care way more about privacy than expected' part resonates. Building something privacy-first out of Berlin I see it constantly. Once people actually read what most calendar tools do with their data the reaction is always 'wait, they store this?'. Hetzner Germany is the right call for that audience.
Totally — the "wait, they store WHAT?" moment is real. Most people don't even think about it until you spell it out. And honestly, GDPR compliance turned out to be a competitive advantage, not just a legal checkbox. Are you building something in the privacy space too?
Oh man, the bidirectional loop bug made me laugh... I can just picture the event count going up exponentially 😂 Glad you caught it early!!!
I'm actually dealing with the Google/outlook sync problem right now.
Studying and working can often be difficult to coordinate🥲
Going to check this out!
Haha yeah, it was one of those bugs where you stare at the logs and go "why are there 400 events in a calendar that had 12" 😅
And that's exactly the use case — uni calendar on Google, work/internship on Outlook, and suddenly you're double-booked. DM me if you want to get set up, happy to help!
For getting beta users: what worked for me was being active in communities where the target users already hang out (Reddit, Slack groups, etc.) and just answering questions about the problem before ever mentioning the product. Took a few weeks but the users you get that way are way more engaged than cold signups.
Rooting for you — this is a real pain point for anyone working across orgs.
Thank you man! I'll try it