delta change per month

I have a fairly small table

On each row is a different date, e.g.  [4/1/2015, 5/1/2015, 6/1/2015]

On each of these rows there is a value, lets say, [7, 8, 15] respectively for above

I want to find the change per month as a column e.g.

ChangeFromPreviousMonth = [ n/a, 1, 7 ]

n/a because there was no data point before 7 in the above example

Any thoughts on how to achieve this would be greatly appreciated! Thanks!