I found a nice question on the MSExchange forum which I thought it may be interesting. It may happen that during an import there goes something wrong with the calender,  how can you clear all items from the calender ? The answer is via the Powershell with the following command:

Export-Mailbox –Identity "mailbox" –PSTFolderPath "path to PST" –IncludeFolders "\Calendar”,”\Agenda” –DeleteContent –Confirm:$False


Johan Veldhuis