RESTORING iCLOUD CALENDARS FROM TIME MACHINE OR A BACKUP

The iCloud Calendar  and Time Machine don't play nice together. If you accidentally delete your calendars,  they are deleted on iCloud: you restore the cached versions from TM, and iCloud promptly brings them up to date by deleting them again.
 
However it is possible to restore from Time Machine (or another backup system if it's included your calendars) by following the following steps:
 

• Turn off your internet connection. Make sure that it really is completely off.
• Restore from Time Machine or your backup. This should restore your calendars to list under your iCloud username.

 
Now carry out this process with each calendar:
 

1. Select a calendar listed under your iCloud login name.
2. From the File menu choose Export...>Export. An .ics file will be saved at your designated destination.
3. Create a new calendar with the same name as the one you've just exported from, choosing 'On My Mac'.
4. From the File menu choose Import... Select the .ics file and choose to import it into the calendar you just created (make sure you don't import it into the iCloud version).

 
Now your calendars are listed under 'On My Mac' as well as under your iCloud login name.
 
Reconnect to the internet. Your iCloud calendars will disappear as before. Now carry out the following process with each of the 'On My Mac' calendars:
 

1. Select your 'On My Mac' calendar. From the File menu choose 'Export...' then 'Export...'; from the sub-menu. This will create an .ics file at the location chosen in the Export dialog.
2. From the File menu choose 'New Calendar' then your iCloud login name in the sub menu.
3. A new calendar called 'Untitled' will appear in your sidebar under your iCloud login name. Change its name to that of the calendar you exported.
4. From the File menu choose 'Import...' then 'Import...' from the sub-menu. A navigation pane will open: navigate to and choose your exported .ics file.
5. iCal will ask you where to import this to. Click the drop-down menu and select the iCloud version of your calendar (make sure not to choose the 'On My Mac' version). Import.
6. Your calendar contents will appear in the iCloud version of the calendar but this may take some time. You need to leave iCal running until the rotating icon at the top of the sidebar disappears.
7. You can now delete the 'On My Mac' version of the calendar (control-click it in the sidebar list and choose 'Delete').

 
You should back up your calendars periodically by exporting the .ics files as in step 1 above and keeping them safely.