Values of Date and DateTime fields are not set in OLE documents (for example, .doc, .xls, or .ppt files) if the time zone of the client machine is greater than UTC+1. If these Date or DateTime fields are declared as mandatory, then you will not be able to save document changes.
To resolve this problem, you need to either set Date and DateTime as optional fields, or ensure that the time zone is not greater than UTC+1.