SQL & PL/SQL

I've read some of the other threads on the this tope, but can't find a resolution.

My query is supposed to get the last values for the given two days (start date and end date) and I return the difference between the two values. The query as it is only works for days where there is an entry for 12:00 pm. On some days, it will therefore return nothing. But I'm not sure how to get it to choose the last value for the given day and still remain a fast query.

Here is a sample of the query with dates that return a result.

SELECT max_query.sgnl_id as sgnl, max_query.max_val - min_query.min_val result_val
   FROM
  (SELECT t1.sgnl_id, t2.smpl_dte, t1.pv_id,
    MAX(t2.integ_since_val) AS max_val
     FROM diag.blm_acct t2, diag.pv_sgnl_id_assc t1
    WHERE t2.smpl_dte = TO_DATE ('11/15/2008 12:00:00 PM', 'MM-DD-YYYY HH:MI:SS AM')
    AND t1.pv_id = t2.pv_id
  AND t2.pv_id        IN
    (SELECT pv_id
       FROM diag.pv_sgnl_id_assc
      WHERE sgnl_id IN
      (SELECT sgnl_id
         FROM diag.series_sgnl_rec_asgn
        WHERE series_id = 'Series5'
      )
    )
 GROUP BY t1.sgnl_id, t2.smpl_dte, t1.pv_id
  ) max_query  ,
  (SELECT pv_id,
    MAX(integ_since_val) AS min_val
     FROM diag.blm_acct
    WHERE smpl_dte = TO_DATE ('11/10/2008 12:00:00 PM', 'MM-DD-YYYY HH:MI:SS AM')
  AND pv_id        IN
    (SELECT pv_id
       FROM diag.pv_sgnl_id_assc
      WHERE sgnl_id IN
      (SELECT sgnl_id
         FROM diag.series_sgnl_rec_asgn
        WHERE series_id = 'Series5'
      )
    )
 GROUP BY pv_id
  ) min_query WHERE max_query.pv_id  = min_query.pv_id
   ORDER BY max_query.sgnl_id

The data set in the table is quite large, but the above query returns only 49 results (one for each signal in 'Series5').

I tried the following statement by itself to get the last value, but it takes quite a long time to execute (increased the overall query time from 1 sec to 400+ sec when used in the query above):

SELECT DISTINCT trunc(smpl_dte) as the_day, 
         first_value(integ_since_val) over (partition by trunc(smpl_dte) order by smpl_dte desc) as last_val_per_day
  FROM diag.blm_acct WHERE pv_id = 3 and trunc(smpl_dte) = to_date('05/18/2008', 'MM-DD-YYYY ')

Any thoughts?