code: def date_range(col, start_date, end_date, args, block)
          operation = "   Date.strptime(\"#{start_date}\",  \"#{date_format}\") "    +
                      "<= Date.strptime(\"[value]\",        \"#{date_format}\") && " +
                      "   Date.strptime(\"[value]\",        \"#{date_format}\") "    +
                      "<= Date.strptime(\"#{end_date}\",    \"#{date_format}\")"
          pivot["#{start_date}-#{end_date}"] = { col: col, operation: operation }
        end
docs: Adds a date range filter