it appears that this is a multiple row calculation so what you are trying to do is calculate the difference between a min and max value or only on specific dates?

I am almost at a solution, but can't quite get it to work right.

I created a formula field titled DIFF as the following:
if {table.meter_id}= Previous({table.meter_id}) then
({table.reading}) - previous({table.reading}) else
0


When I place this field in the detail section of the report, I get the difference between two fields, but the calculation is returning the difference in the previous row instead of the most recent row.

Example:
DEVICE / READ / READ DATE /    DIFF
1     /   8500 / 12/21/14    /   0
1     /   8000 / 11/18/14    /   500
1     /   7650 / 10/19/14    /   350

2     /   6500 / 12/26/14    /    0
2     /   6300 / 11/19/14    /   200