Currently, there is no way to 'set it and forget it' for a Custom Date Range. The belief is that you can select Start of Current Month minus 1 Month and End at Current Day minus 1 Month. This results in subtracting the full month and not the 28/30/31 days. So, if I were to try to calculate Prior Month To Date, I would end up with the date range containing only the first of the previous month - not 1-12 or whatever.
There is a workaround that I can subtract 28/30/31 days from the above calculated date range but I have to edit it every single month. This is not ideal.