I tested the problem one more time. The conclusion that the imported date is given when creating a export of same imported file is FALSE. The date is also not given in the export file! Because I could not wait, I looked into the code a detected two bugs:
$book_class->release_Date = mosBooklibraryImportExport::getXMLItemValue($book,'releasedate'
change 'releasedate' to 'releaseD
change "releasedate" to "releaseDate"
The reason I changed the letter d to D is because of the code that is written for creating a XML exportfile:
Look at the file "booklibrary.class.php". See line 342. I used the export file output to define my importfile. In code they are different!
So we could also change the error in this file, so that the XML import and export xml templates are the same. Maybe this option is the best one
I think this is the real bug.