Hi Toralf,
That migth be problematic. I am not sure, how those dates are being stored, but I am guessing it is probably some sort of a serialized object storage (maybe in the post meta table?). Creating date filters is possible for single date custom fields, but I don’t think that will work with this case, as it is much more complicated.
Usually I recommend this service or Envato Studio to search for developers for customizations.