You can use a relationship that refers to all related records with a date one month less than either the current record's date or the current calendar date from your computer's system clock.
The trick is to use a calculation that returns the same value for all records of the same month and year.
DateField - Day ( DateField ) + 1
Will return the date for the first day of the month for all dates in that month.
Date ( Month ( DateField ) - 1 ; 1 ; Year ( DateField ) )
Will return a matching date for the previous month.
Substitute Get ( CurrentDate ) for DateField in the last calculation to get a matching value for the month preceding the current month as shown on your system clock.