You can have a SUMMARY field on the field which contains sales. Do a find on the date range to pull the months. If you do a GETSUMMARY field that does it at the record level, it will be on every record for the given month.
You could also do a table of summarized sales. Each record in your sales table would become a child record to this table. One record in this new table could be sales by week that you summarize for a month using a sub-summary part. Or you could just have one record per month that has a calculation field to sum on the records for the month. There are several options dependent on how your data is laid out in the existing table and how you want to do reports.
Interesting.... thanks so much, I will see what I can do.
I take it there is not simple way to come up with a calculation field that would be able to parse this data from the sales table?