Currently, when you run a report with the parameter "Last Month" equalling 1, it pulls the last 30/31 days from the date of the report execution.  I suggest/request that it be updated to the last calendar month, as in all of July or August, etc.  If the user/customer wants the last 30/31 days, he/she can still use the parameter "Last Days" equalling 30 or 31.