Sorry to hijack your thread but I notice that you have V8. Things have moved on since then and more options have opened up for users of later versions so I thought it would be worth mentioning a few more options.
The Age() and DateAdjust() function might be another way to make this calculation in ways that take into account the number of days in a month. There are also now 'standard' functions (of the user defined type) to find. inter alia, the end date of a month or the start date of a month and thus, for example, quickly ascertain how many days there were in last month and use the answer to set the cut off date, perhaps with a user specified tolerance factor (e.g 1 month plus 3 days) for selections based on ageing ...
You can do most of the same things in V8 (and earlier versions) but you need to create the formulas for each stage if there is no suitable function to do it for you.
There's nothing like building filter formulas in the morning to start the day off right...
Age() is much more elegant, isn't it?