Queries not always using index on timestamp search

Here are two queries (under 7.4.1):

'mytime' is a timestamp field that is indexed.

select * from mytable where mytime > '2004-06-21'

select * from mytable where mytime > current_date-3

Looking at an explain on these queries, the first one will use the
index and the second one will not, even though 'current_date-3'
produces the same date as the hard-coded one in the first search.

Is there a way to get the second query to use the index?
--
Mike Nolan