Why does a simple query not use an obvious index?

    select count(*) from billing where timestamp > now()-60

should obviously use the index

    CREATE INDEX billing_timestamp_idx ON billing USING btree ("timestamp" timestamp_ops);

on a table with 1400000 rows.

But it uses a Seq Scan. If I set enable_seqscan=no, it indicates a queryplan could not be calculated.

Why does this simple query not use the timestamp index, and how can I get it to?

Thanks, Jack