You can set up a script to perform a find or do a manual find (cntrl-F or command-F depending which platform you are using). In the find mode, the <Date> field, you can manually insert the date range desired (i.e. 8/01/04...8/15/03). This will return all the invoices on and between those two dates. You should have a summary field of the Invoice Total-called Sum_Report1, that way, when you do a find with a date range of 15 days, you will then have the Sum of all invoice $$$ for that particular date range. You would approach collecting the Report2 information the same way.
I'm not sure how you want to present this information, but I am assuming you want it recorded. I would use the SetField function to place that information from the Sumfield into the field you so choose, such as: Aug_Inv_Rpt1 and Aug_Inv_Rpt2 respectively. You'll have to do this for each month, but it can be automated with a little work.