When a zipfileset has a prefix attribute, a directory entry is added to the archive. The issue is that the date of this entry is always the current date. That means that if the task is run on the same set of files, it will not produce the exact same archive. I suggest that the prefix directory (and its parent directories) is created with the date of the most recent entry at the root of the fileset, or with the current date if the fileset is empty.