I run Sunbird from a dedicated data partition and run several XP's from the same HD using bootitng partition and boot manager.
Suddenly Sunbird couldn't remember the hour being set in a new appointment and would always change it to 11:00.
I found that this only happened from one of the XP's.
I narrowed it down to this:
It happens if the Windows Clock's "Automatically Adjust Clock for Daylight Savings" check box is unchecked!
Hard to believe. At least checking it again fixed the problem.
I don't know if the non-portable Sunbird has this sesitivity, if Microsoft is doing something wrong, or what.