Formula for finding the difference between the 1st of month and 1st of another month

Good Afternoon,

Need some help with a formula. I want to find the sum of two rows (6 columns) but I need it to search for the 1st of the month of each two months.

See attached worksheet.

So if I have a date in cell O2 (01/01/2014) I need a formula that sums the row that contains 01/01/2014 and subtracts it from the sum of the row that contains 02/01/2014.

I'm using this formula [=(SUM(D35:I35)-SUM(D4:I4))] for the total used in the month but I want it to update month by month. I attempted using [=(SUM(IF(MONTH,DAY(A6:A39)=MONTH(O2),D4:I39)))-(SUM(IF(MONTH,DAY(A6:A39)+1MONTH,DAY(O2),D4:I39)))] but cannot resolve the date issue.

Thanks,