GTAR incremental backup problem

[sevelm1 . edu]

Hello,
    version of gtar I'm currently using is 1.13. My problem is following:

    I have two directories
        $HOME/sct/backup/logs_FSS
        $HOME/sct/backup/tmpdir

  Now:

   $> set DAT = `date`; echo $DAT
        Thu Jul 19 07:28:03 MEST 2001
   $> cd $HOME/sct/backup
   $> ls -l tmpdir
         ............                          30 Jul 19 07:26  date.out
         ............                          69 Jul 19 07:27  date.out
         ............                          86 Jul 19 07:28  date.out
   $> gtar -c -v -G -N "$DAT"  -f /tmp/incr.tar .
         ./
         ./who.out                      ---- this is OK

   $> cd ../logs_FSS
   $> gtar -c -v -G -N "$DAT"  -f /tmp/incr.tar .
       ./                                      ---- this is OK

  This is still ok, because  logs_FSS contains files older than $DAT.
   The problem is here:

  $> cd $HOME/sct/backup
  $> gtar -c -v -G -N "$DAT"  -f /tmp/incr.tar .
   gtar: Directory ./tmpdir is new                          ???????
directory is older than $DAT
   gtar: Directory ./logs_FSS is new                  ???????
directory is older than $DAT
   ./tmpdir/
   ./logs_FSS/backup_1.tgz                                 ???????
file  is older than $DAT
   ./logs_FSS/backup_2.tgz                                 ???????
file  is older than $DAT
   .
   .
   .
   and so on

Some files are older than $DAT and they are still in archive. What's
meaning of parameter -N "$DAT"?????

I have specified -N "$DAT" and I want only files newer than $DAT appearing
in archive.

Can you help me with this problem?

                              best regards
                                                   Mirek Sevelda