Need Help With Job-duration Query

I have a query that displays number of hours duration in a jobs by months matrix. Here is a simplified version of my tables.

CREATE TABLE jobhead (
jobnum varchar (14) NULL,
jobcompletiondate datetime NULL
)

CREATE TABLE labordtl (
jobnum varchar (14) NULL ,
clockindate datetime NULL ,
clockintime decimal(6, 2) NULL ,
)
Here's an abbreviated query.

SELECT h.JobNum,
(CASE WHEN MONTH(h.JobCompletionDate) = 1 THEN datediff(day,MIN(l.ClockInDate),MAX(l.clockInDate)) ELSE 0 END) AS JAN,
(CASE WHEN MONTH(h.JobCompletionDate) = 2 THEN datediff(day,MIN(l.ClockInDate),MAX(l.clockInDate)) ELSE 0 END) AS FEB,
...
FROM JobHead h
INNER JOIN LaborDtl l ON h.JobNum=l.JobNum
WHERE JobCompletionDate>='20070101' AND JobCompletionDate <'20080101'
AND l.ClockInTime<>0
GROUP BYh.JobNum
,h.JobCompletionDate

The query shows, for each job, the month in which the job completed, and the number of hours it took to complete. I€™m calculating the number of days€™ duration by doing a datediff between the oldest and newest clockindates. I need to ignore adjustment transactions in the labordtl table €“ these rows are easily identified as they have clockintime values of 0. So far, so good. Now here€™s my problem.

There are some jobs which have only one €œreal€? labor transaction €“ this could happen if the job only took one day to complete. Other labor transactions may exist for that job, but let's say they are adjustments which we can ignore -- the date they were entered should not extend the duration of the job. In this situation, my datediff between the oldest valid transaction and the newest, returns 0. I don€™t have to count hours between clockintime and clockouttime. The rule is simply that if there is only one "real" labor transaction, I need to count this as a 1 day job.

I thought a nested CASE statement or expression might be the way to go but I didn't make any real progress.

Any ideas to solve this problem would be appreciated.