indexing datetime - Mailing list pgsql-sql

I have a very large and fast-growing table with a datetime field. Most of my
queries are based on date comparison as well. Like:

select * from table where time > 'Aug 1 0:0:0 1998' and time < 'Sep 1 0:0:0
1998';

How can I index the table to optimize my queries? Or maybe datetime is a bad
choice for this?

-- Sergei