Problem with Dates

I am using 7.0.3,
I have a column tstamp defined to be 'date';

With a current value of '31-12-2000',
if I update tstamp=tstamp+'1 year'::timespan
I get '1-1-2002'

Is this what is supposed to occur.
If this isn't the right way to do this, how should it be done ?

Thanks
-- 
Glen and Rosanne Eustace,
GodZone Internet Services, a division of AGRE Enterprises Ltd.,
P.O. Box 8020, Palmerston North, New Zealand
Ph: +64 6 357 8168, Mobile: +64 21 424 015