Speeding up a query

Greetings.

The following query returns the last names of people in our database whom
we consider 'students' during the time period 1-jan-1998 to 1-jun-1998.
It's based on selecting events from a table in which each event can
determine that a person is a Student, NotStudent, or Neutral; the idea is
that a given person is a student from the date of their first Student
event to the date of their next NotStudent event, etc.  

The problem is that it's very slow - on the order of 45-60 seconds to
generate a list that's only about 30 names long.  Can anyone offer some
advice on speeding it up? The dates need to be user-selectable.

Many thanks.

select last_name from personal_data where id in (
select distinct id from event_detail, event_universe where
event_detail.eventtypeid = event_universe.eventtypeid and
event_universe.studentstatus = 'Student' and
event_detail.date_effective <= datetime(date('1-jan-1998')))
and
id not in (
select distinct id from event_detail, event_universe where
event_detail.eventtypeid = event_universe.eventtypeid and
event_universe.studentstatus = 'NotStudent' and
event_detail.date_effective <= datetime(date('1-jun-1998')))


---------------------------------------------------------------------
Andrew J. Perrin - aperrin@demog.berkeley.edu - NT/Unix Admin/Support
Department of Demography    -    University of California at Berkeley
2232 Piedmont Avenue #2120  -    Berkeley, California, 94720-2120 USA
http://demog.berkeley.edu/~aperrin --------------------------SEIU1199