-o    Set .ZIP file date to oldest file in .ZIP file
  -o-   Set .ZIP file date to current date (used to override
          configuration file)

  Normal default operation for PKZIP is to set the date of a .ZIP file it is
  modifying or creating to the current date.  By specifying the -o option
  you can tell PKZIP to set the date of the .ZIP file equal to that of the
  oldest file within the .ZIP file.

  This option may be made the default, see PKZIP Configuration.

  If this option is set, you may override it by using the -o- option.  This
  will force PKZIP to set the .ZIP file date to the current date.  Note that
  the -o- option must be completely separate from any other options, such as
  the -f in the example:

     pkzip oldfiles.zip -o- -f c:\wp51\text\*.wp