-a+   Turn off archive attribute of added files

  By using this option you can create a complete backup of your disk, while
  clearing the archive attributes to make the way for incremental archiving.

  Incremental archiving makes use of the archive attribute to take only the
  files which have been modified since the last backup.  In order for this
  process to work smoothly you must first have a complete backup and a
  clearing of the archive attribute for all files.

     C:>pkzip f:backup.zip -a+ -rp