If you use calendar views this might shed some light on strange happenings.

Basically if you have a calendar view that has the first column sorted on a date field then the documents show up on the correct day.

I have found that changing the calendar style from month to week or whatever caused the documents to appear on other days, one day out as well as the original date, when the date hadn't changed.

This was found to be because the date needs to be stored as a Date/Time field rather than just a Date field. Soon as this was done the calendar worked as required.

ie instead of "20/09/1969" change it to "20/09/1969 00:00:00".

This is actually specified in the help, but it is a pedantic requirement that is often forgotten.