Three account routes
Apple-managed calendars arrive through the system calendar store. This includes iCloud and other accounts already configured for Apple Calendar. Google and Microsoft 365 can also connect directly under Settings → Accounts.
You can use the sources together. InstaCal groups calendars by account and provider and attempts to de-duplicate the same event when it is visible through more than one route.
Refresh behavior
System calendars refresh through Apple’s calendar framework. Direct accounts use provider APIs and incremental sync. On Mac, Settings → General → Refresh accounts every controls the normal interval; manual refresh remains available when you need the newest state immediately.
Offline and cached events
Direct accounts retain a local event cache so a temporary network failure does not empty the calendar. Accounts labels the connection Offline while showing the last known data. Changes that require the provider wait until connectivity returns.
Sign-in needed
If a provider expires or rejects authorization, the account shows Sign-in Needed. Open it under Settings → Accounts and reconnect. Removing and re-adding should be a later step because it discards the local cache and account-specific choices.
Avoid duplicate account paths
If the same Google or Microsoft calendar is both enabled in Apple Calendar and connected directly, compare the account labels in Settings → Calendars. Hide one copy when de-duplication cannot prove that two differently identified items are the same.
