monthly report possible?

How do I define the filters if I want a monthly report on all successful orders?

First I have 'Start time was accessed IN LAST 31 days' and linked it with AND with 'Order status = Completed (successful)'.

But then I get a report in which the same order is listed again and again every second.

No, I only want one line per order.

My field definition:

Start time, order name, total size of data, order duration, order status, error code, error category, error text.

Please remember that a month sometimes has 30 days and sometimes 31 days.