I assume you want this on a regular Seblod list... I think it would be easy if there were an option to calculate the date range. Like it was possible in this Seblod v2 live plugin. As far as I can see, there is no date math option in SearchForm -> Button#3 Match. There are only options for "past" and "future" dates. So, no configuration to set a limit or create a maximum future date.
So, it seems you would need custom PHP code or a Variation to manipulate the date in searchForm.
--- EDIT ---
Calendar fields and date math is possible and build-in !!!
I guess, the product "Code Pack" is quite save and very cool. Thinking about a purchase ;)
Not sure about custom "Variations" which can be exported in Seblod -> Templates -> Variations Button in upper right corner. Not clear how to deal with them, except for the ZIP download. They can't be installed separately and usually come with a Seblod template installation.
OT & PS: Just had a quick look on the site and some accessable pages. Seems to have a lot of content, listings and relations. I really wonder that the site runs on J!1.7 (you might remember that looooooong rocky road from 1.5 > 1.6 > 1.7 > 2.5! :( ) I guess, "Drupal" would have been the better and smarter choice for such task. Even an older version ;)