def parse(value)
      unless value.is_a?(String)
        raise TimeParseError, "value must be string: #{value}"
      end

      if @cache1_key == value
        return @cache1_time
      elsif @cache2_key == value
        return @cache2_time
      else
        begin
          time = @parse.call(value)
        rescue => e
          raise TimeParseError, "invalid time format: value = #{value}, error_class = #{e.class.name}, error = #{e.message}"
        end
        @cache1_key = @cache2_key
        @cache1_time = @cache2_time
        @cache2_key = value
        @cache2_time = time
        return time
      end
    end
def parse(value)
      unless value.is_a?(String)
        raise TimeParseError, "value must be string: #{value}"
      end

      if @cache1_key == value
        return @cache1_time
      elsif @cache2_key == value
        return @cache2_time
      else
        begin
          time = @parse.call(value)
        rescue => e
          raise TimeParseError, "invalid time format: value = #{value}, error_class = #{e.class.name}, error = #{e.message}"
        end
        @cache1_key = @cache2_key
        @cache1_time = @cache2_time
        @cache2_key = value
        @cache2_time = time
        return time
      end
    end
TODO: new cache mechanism using format string