code: def date(col, comparator, date, args, block)
          comparator = '==' if comparator == '='
          operation = "Date.strptime(\"[value]\", \"#{date_format}\") #{comparator} " +
                      "Date.strptime(\"#{date}\", \"#{date_format}\")"
          pivot["#{comparator}#{date}"] = { col: col, operation: operation }
        end
docs: Adds a date filter