I decided to post this issue because I did not find out any wrong in my system, and if it is a WAD or Bug, please confirm this topic is correct and I will send this link to my customer.
Problem:
Install SharePoint 2007 with the default settings for everything. Create a celendar list and then create a column named "My Date Time" or something you want with DateTime type and show both Date and Time.
Create an item in Calendar with All Day Event checked, type a date into "My Date Time"
After save, everything is fine. Please see the picture in view form.
Title, AlldayEvent, My Test Datetime show correctly, data kept. But what is happening when I edit this item?
"My Test Datetime" was wrong, the date is minus 1 day and if I save this item, the data will store this date.
There is very strange that it will not change for the next editing. And this issue does not happen in SharePoint 2010