Hour report - time frame filter

Each month I produce a report from data captured from the last calendar month. I thought I had set up the report so that I could reference it at any point in the following month and the data shown would be from the previous calendar month. I have attached a screen shot of the filter I have been using in a hour report, but now thinking that $$TODAY-1m actually means 1 month ago today. How do I make it be the end of the last calendar month without putting in the actual dates of the last calendar month - I need this to be a rolling report = low maintenance!

Thanks for any help and hope this makes sense!