Using SELECT WHERE

I made a database and I would like to select entries
which have data not equal to the specified date.
I have postgresql-7.3.4-11  on Red Hat Fedora Core 1.

Here is the command

RabStudies=> SELECT RabNo, ImplantDate, Comments    FROM RabStudiesInfo
where  implantdate <> 0001-01-01;

and I get

 rabno | implantdate |                                     comments

   307 | 2004-02-23  |  No mimetic No target
   309 | 2004-02-23  |  No mimetic No target; Frame 12 study terminated
   314 | 2004-02-27  |  No mimetic No target
   311 | 2004-02-27  |  No mimetic No target
   310 | 2004-02-26  |  No mimetic No target No decoy
   315 | 0001-01-01  |  No Tumor
   320 | 0001-01-01  |  No Tumor
   316 | 0001-01-01  |  No Tumor
   313 | 0001-01-01  |  No Tumor

Why entries with implandate are listed?
It is the same when I use != instead of <>.

Thanks,

Michal