Since you're using tables, you can create a SQL Expression to help with this calculation.
I'll call the SQL Expression {%CurrentMonth}. The code for it is: Trunc(Sysdate, 'mm'). This will give you the first day of the current month.
From here, you create running totals (unlike summaries, you'll only be able to use these in footer sections). In the Evaluate section, select "Use Formula" and enter something like the following in the formula (use the actual date field from your data): {table.datefield} < {%CurrentMonth}. Your mins and max's should now calculate only for months prior to the current one, but your data for the current month will still display on the report.
Another way to do this using summaries (if you need the data in a group header) is to create a formula like this:
if {table.datefield} < {%CurrentMonth} then {table.numberfield}. You can use this formula in summaries.
-Dell
A computer only does what you actually told it to do - not what you thought you told it to do.