From: DeskNow Knowledge Base
http://www.desknow.com/kb/

How do I restore a calendar (events and tasks) from a backup?
http://www.desknow.com/kb/idx/9/134/article/
This article applies to DeskNow 3.1 and later

DeskNow by default performs a nightly backup of all users' calendars. This is configured in the Administration/Backup page.

Calendars are backed up (within the configured backup folder) into the file usercalendars.tar.gz . tar.gz archives can be opened with most unarchiving programs, including WinZip in the Windows world, and obviously the tar command in Unix/Linux/Mac.

To restore all events and tasks for a calendar

From the usercalendars.tar.gz archive, extract the file domainname/username/calendarname.ics

Then login into DeskNow as the user account, open the calendar, and select Folder/Import . Then simply upload the .ics file extracted from the backup.

DeskNow will normally not change items that were already present in the calendar, and will avoid duplication. The only exception is for old items created in early releases (<2.7). In this case, you may want to consider clearing the calendar first, or importing into an empty calendar to avoid duplication.