Same stores filter - dynamic column in table?

Hi there,

I would appreciate any ideas or help on the following case: I have a table with tickets, in which sales are registered with date, amount and the store where the sale was made. This table is related to another one with the store catalog table, with attributes for each store, including opening date. In my report I have filter for dates and I want to include a filter for "same stores" (stores that have been opened by more than 18 months) to be able to compare year over year growth, the catch here is that a "same store" depends on what dates are filtered in the report (minimum date filtered would have to be greater than the store opening date minus 18 months). I was thinking on including a "same store" variable column in the store catalog table, which would change depending on the dates selected by the report, although I don't know if this is possible... any ideas?

Thanks