We have noticed several issues with the datetime attributes. It has also been subject in the forum. Sometimes dates are changed unexpectedly because of localizing and other mechanisms, local deployment differs from server behavior etc. Is it an idea to make a type Date that is not subject to localization because is it simply a date. (Yes I can file a feature request, but I would like to know your opinion first.)
Chris de Gelder
In the attribute edit form there is a check box 'Localize' that you can uncheck if you do not want localization of dates.