The tool to recover iPhone data and extract contacts, messages, calendars, recordings and photos from iOS backups created by iTunes or other applications.
Extract and Export Calendars and Events from iPhone Backups
Learn how to install and run iBackup Viewer

With iBackup Viewer you can export events from all calendars, or just single calendars extracted from iPhone backup.
Click the "Calendar" icon after opening a backup to load calendars and events.

Click on the "Export" button to show options

Click on individual event will show details about the event, click "Export" button to save the event to ICS file.