code: def guess_date_format
        begin
          line = @data.gets
          break if line.nil?
  
          date = line[1..-1]
          guessed_format = Qif::DateFormat::SUPPORTED_DATEFORMAT.find { |format_string, format|
            test_date_with_format?(date, format_string, format)
          }
        end until guessed_format
  
        @data.rewind
  
        guessed_format ? guessed_format.first : @fallback_format
      end
docs: Guess the file format of dates, reading the beginning of file, or return nil if no dates are found (?!).