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