I'd use sub-summary parts for the sub totals and place the summary fields in this part.
To group the records by month so you get monthly sub-totals use the following date calculation:
DateField - Day(datefield) + 1
set it to return date. This calculation returns a date = to the first day of the month for every date within that month.
Use the above calculation field as your break ("sorted by") field for your sub summary part and include this record in your sort order.
Why not sort on a subsummary field 'Month' add year to sort Year with the subtotal summary field in it in it and the MonthName.
This gives me the months total.
That's exactly what the above calculation field enables. Since it is still a date field. Sorting on this field will also group your records in chronological order and can be formatted on a layout as Month Name, Year for reporting purposes.
Thanks Paul! That worked great.