How to calculate the difference between values in a custom PSObject?

Question

If I have a PSObject (created from an imported CSV) that has data like the following:
date,Total size of files uploaded
03-17-2014 09:08,1293334574547
03-17-2014 09:10,1293335326912
03-17-2014 09:13,1293336523562
03-17-2014 09:14,1293337541156
03-17-2014 09:16,1321031223511
03-17-2014 09:16,1321032156844
This CSV will have lines added to it, so the total number of lines will vary.
Is there a way that two most recently added values and calculate the difference of the "Total size of files uploaded" values? 
I have the columns properly cast as datetime and
int64 already using this:
$csv = Import-Csv $csvLocation |
Where-Object { ![string]::IsNullOrWhiteSpace($_.datetime) } |
Foreach-Object {
$_.datetime = $_.datetime -as [datetime]
$_."Total size of files uploaded" = $_."Total size of files uploaded" -as [int64]
$_ }
 I tried searching thinking surely there must be someone else who has tried to do this" but I cannot find anything. I am just not sure how to approach this problem. Any help would be appreciated!

Answer 1

Assuming all of your records are sequential with the last two being the most recent you could use this:
$csv = Import-Csv $csvLocation |
Where-Object { ![string]::IsNullOrWhiteSpace($_.datetime) } | select -last 2
Foreach-Object {
$_.datetime = $_.datetime -as [datetime]
$_."Total size of files uploaded" = $_."Total size of files uploaded" -as [int64]
$_ }
$Difference = $csv[1]."Total size of files uploaded" - $csv[0]."Total size of files uploaded"
This uses the index of the imported array and just the last two records.  Note that this will give a negative value if the total is less in the later record.  Hope this helps!
You could also use 
| sort datetime -ascending | select -last 2
if they aren't already in order...