I had a calendar at every client and one of my own, and someone always booked over something they couldn't see. SYNCDATE fixes that for people whose week runs across several companies.
Connect Google Calendar, Outlook or Microsoft 365, iCloud, Fastmail, Nextcloud or any CalDAV server. Pick which calendars feed which, one way or both ways. Book a meeting at one company and the time is blocked at the others, usually within a minute on Google and Outlook.
The part I built it for is what each copy says. A client calendar shows Busy and nothing else. Your own calendar keeps the real title. A copy can read "[Acme] Standup" so you know where it came from. Plus adds if/otherwise rules; Pro gives each calendar in one sync its own rule.
Every run is on the dashboard. Delete a sync and it can remove every copy it made; your original events are never edited.
Free for two calendars, no card, no time limit. Paid plans add accounts, calendars and rules, with a 7-day trial. Built and hosted in the EU by one person.