select with date_part and month failure....

Hello

I have the same problem against various PostgreSQL 8.1.3, 8.0.2 and 7.4.7:

I have a table containing periodical invoices. it contain's the last time the
invoce was printed and a period in month to be waited before next time.
My querry works with date_parts on days but never with months:

create temp table tstdates (
lasttimedone date,
period integer
);

insert into tstdates values ('2006-01-01',2);
insert into tstdates values ('2006-02-01',2);
insert into tstdates values ('2006-03-01',2);
insert into tstdates values ('2006-04-01',2);

-- doesn' t works :
select * from tstdates where
date_part('month',now()-lasttimedone) >= period;

-- works :
select * from tstdates where
date_part('days',now()-lasttimedone) >= period*30;

Can someone point me an error ?

T.I.A. PM.