BusyCal has the ability to view and manage multiple calendars across multiple accounts and systems such as Google Calendar, Apple iCloud Calendar, and Microsoft Outlook Exchange.
One of the biggest advantages of BusyCal is that it lets you move all calendar entries from one system to another. For example, if you decide to migrate from Google Calendar to the Apple iCloud Calendar, you can easily click, drag, and copy all calendar entries from Google Calendar to the Apple iCloud Calendar.
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Source: ResourcesForLife.com – BusyCal Calendar Software for Synchronization Across Multiple Accounts and Systems: Google, Apple, Exchange