Min date after date from different dimensions

[Flint89]

I am trying to calculate the 'LT Hour' lifetime of a part. 

To do this, I am taking the most recent LT Hour value which is in the same table as the LifeLoggingDate and then subtracting the oldest LT Hour value, but only when it exceeds the date in which the part was installed (this information is in a different table to the lifelogging). 

The below formula works when I select a PartSN in the table. 

aggr(Max({<LifeLoggingDate={"=aggr(max(LifeLoggingDate),DeviceID)"}>}LTHours),PartSN,DeviceID)
-
aggr(Min({<LifeLoggingDate={"=aggr(min(LifeLoggingDate),DeviceID)"},LifeLoggingDate={">$(=(Install_Date))"}>}LTHours),PartSN,DeviceID)

My intention is to see all Part SN and be able to sort by the LTHours to assess part lifetime.  Any suggestions?