I have a table like this:

Index , PersonID  , ItemCount         , UnixTimeStamp
1     , 1         , 1             , 1296000000
2     , 1         , 2             , 1296000100
3     , 2         , 4             , 1296003230
4     , 2         , 6             , 1296093949
5     , 1         , 0             , 1296093295

Time and index always go up. Its basically a logging table to log the itemcount each time it changes. I get the most recent ItemCount for each Person like this:

SELECT *
FROM table a
INNER JOIN
(
 SELECT MAX(index) as i
 FROM table
 GROUP BY PersonID) b
ON a.index = b.i;

What I want to do is get get the most recent record for each PersonID that is at least 24 hours older than the most recent record for each Person ID. Then I want to take the difference in ItemCount between these two to get a change in itemcount for each person over the last 24 hours:

personID ChangeInItemCountOverAtLeast24Hours

    1        3
    2        -11
    3        6

Im sort of stuck with what to do next. How can I join another itemcount based on latest adjusted timestamp of individual rows?