Why we built Caltsu

I have four calendars: three for clients on Outlook, and a personal one on Google Calendar.

For about two years, my "system" was to check all four before accepting any meeting. It worked until it didn't. The third time I showed up to a client call and realized I'd double-booked it with a dentist appointment, I gave up on the system.

The options weren't great

So I looked at what was out there. The tools fell into two camps.

Camp 1, AI scheduling assistants. Tools like Reclaim.ai and Morgen want to manage your entire day. They find focus time and move your tasks around. They're good at it, if that's what you want. But I don't want an AI managing my day. I just want my calendars to talk to each other.

Camp 2, calendar sync tools. These were closer. But most of them work by syncing your actual event details between accounts. Your "Therapy: Dr. Korhonen" appointment on Google Calendar gets copied, title and all, to your work Outlook. Now your client can see it. Maybe even the description.

For freelancers and consultants, that's a dealbreaker. Client A should never see that you have a "Project kickoff with Client B" on your calendar. Your team's shared calendar doesn't need to know about your personal appointments.

Some tools let you rename events to "Busy," but it's a setting you have to find and remember to turn on. One missed checkbox and a client reads your therapist's name.

What I actually needed

My list was short. I wanted a tool that would:

1. Sync my busy time across Google and Outlook calendars.
2. Never, ever copy event details between accounts.
3. Sync fast enough that a new meeting blocks off the time on my other calendars before someone else can grab it.
4. Work in the background without me having to think about it.

The third point matters more than it sounds. Tools that poll can take 15 minutes or more to notice a change, and double-bookings happen in that gap. If two people have ever booked the same slot with you a few minutes apart, you know.

So I built it

Caltsu connects to your Google and Outlook accounts. Apple iCloud is not supported yet. When a new event appears on one calendar, it creates a plain Busy block on the others. The block carries the time, a title you choose, Busy by default, and the name of the calendar it came from. It never carries the event's title, attendees, location, notes or meeting link.

It uses push notifications instead of polling. Google and Microsoft tell Caltsu when an event changes, so the sync happens in seconds, not minutes.

There's one plan: €5 per month or €48 per year. You can try it free for 14 days, no credit card needed.

Who it's for

I built it for people like me. Freelancers juggling client accounts. Consultants who keep work and personal apart. Anyone with more than one calendar who's tired of checking all of them before saying "yes" to a meeting.

If you want an AI assistant to plan your day, there are good tools for that. Caltsu isn't one of them. It does one thing. It makes your calendars agree on when you're busy.