issue: Customer complained that Outlook calendar events disappear after some months
2 key things to check as you troubleshoot this:
a. autoarchive setting
b. mobile device synchronization
Note: Outlook does not delete things from the calendar automatically unless you have configured it to do so.
Resolution:
verify autoarchive setting by Right-clicking the Calendar folder , choose Properties. Select the AutoArchive tab and look to see if autoarchiving is enabled. If so, disable it.
If AutoArchive is not the issue, confirm is customer synchs with a hand-held device? That can be a contributing factor.
Mobile devices usually delete the events while the synchronization software ensures event is deleted in Outlook.
Ex-Shell 