** Any ideas on what cause the problem? **
Incorrect local time offset defined for the calendar.
** if it matters, the site's running on Apache and the time server was off by 16 hours for quite a while and it was off by 10 years for a few hours, but that shouldn't have affected manual calendar entries, right? **
I'm pretty sure it would have... if events were added at those times.
Try setting the offset incorrectly again and adding an event. Then set the offset back to were it should be and see what you get for the newly added event?
JMO
Dan O.
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