Timestamp indexes (why ">" or "between" does not use index?)

I have a table with an index on a field called "modify_date".

This works well if I have SQL which ends in

WHERE modify_date = '2008-01-01'

But if I try this condition:

WHERE modify_date = '2008-01-01'

THis index is not used. The EXPLAIN tells me it needs to do a seq
scan. Why is this? How can I make a date/time field index which uses
both equality criteria and the greater than/lesser than/between
criteria?

Thanks.