fix that file modification date is stored in wrong format

(is: zipInfo.dosDate stores seconds from 1980, should be: MS-DOS date time format. references: http://www.pkware.com/documents/casestudies/APPNOTE.TXT (section 4.4.6) and http://msdn.microsoft.com/en-us/library/windows/desktop/ms724247(v=vs.85).aspx)