Customize Date range on report filter

Hi,
anyone knows if is there a way to have a range date formula in a report filter?
I can use the between to select a range, but my range changes every month, so I have to remember to change it.
I'd like to create a formula that says on month1 the startdate is x and enddate Y, and so on for each month, and use this two values in the filter.
(what I'm trying to filter are the Financial Time Phased Data).
Thank you