I think I have found a bug in Sharepoint regarding date fields and calculations. Here is how to replicate:
Make a sharepoint event/calendar list (where items have a starttime and end time)
Make sure your Sharepoint site is on a different timezone than GMT.
Make some entries in the list (some calender events). Make one of the entries a "full day event" where you do not specify the start time. The system then sets the starttime clock to 00:00 which is correct.
So far ... so good!
Now add a column to the list, lets call it "CalculatedStart", make it of type calculated field and of format "Date & Time". Then type in a simple formula like "=StartTime" (or whatever the name of start-time column is in your system language locale).
Now observe that for entries that are full day events (where clock is 00:00), the calculation skews the time by several hours (7 hours for me living in Oslo, Norway).
This IS a BUG, quite serious one too!
Best regards, Eirik