Then in your report make this new field one of your grouping fields with a footer. You add the sub-total fields in the Footer section. You can also chnage the Format Property for the Footer to Force New Page: After section
You don't need to add a field to the query. If you choose the date field in the sorting and grouping options, you will have a choice of grouping by month. If you choose to add a calculated month field to the query, you would also need to add a calculated year field unless your data source contains only one calendar year.