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calendars.txt
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Unfortunately iCal does not the have built-in Time Machine GUI* support that,
iPhoto, Mail and AddressBook have.

* = When you can invoke Time Machine from within the program, you are graphicly allowed to "go back in time".

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However, you can go to the place in the Finder where your iCal calendars are stored.
This will allow you to go back in time to grab a previous version of the calendars.

The calendars are located in ~/Library/Calendars/.
 I'd recommend recovering the whole "Calendars" folder from a previous time,
 rather than the individual calendars because the calendars are rather cryptic in their names.

iCal resources are located in several locations. 
They are:

~/Library/Application Support/iCal/
~/Library/Calendars/
~/Library/Preferences/com.apple.iCal.plist
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