Optimization with dates

Hello,

I really have a problem dealing with dates :

I have a big table (~10M rows) like :

create table gains (
dategain timestamp,
otherfields
);

There's an index on dategain and I want to use it to get the last
30 days records :

explain select count(*) from gains where dategain>=now()-30;

Aggregate (cost=256546.78..256546.78 rows=1 width=0)
-> Seq Scan on gains (cost=0.00..250627.68 rows=2367640 width=0)

whereas :

explain select count(*) from gains where dategain>='now';

Aggregate (cost=27338.47..27338.47 rows=1 width=0)
-> Index Scan using ix_gains_dategain on gains (cost=0.00..27320.71 rows=7103 width=0)

I have tried :
where dategain>=(select now()-30);
where dategain+30>='now'
where date(dategain)>=date('now')-30
and many other, syntactically absurd :-)

Anyone can help me use this index ?

TIA,

--
Jean-Christophe Boggio
cat(at)thefreecat(dot)org -o)
Independant Consultant and Developer /\\
Delphi, Linux, Perl, PostgreSQL, Debian _\_V