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@thefreecat.org                           -o)
Independant Consultant and Developer         /\\
Delphi, Linux, Perl, PostgreSQL, Debian     _\_V