shift_log_period

# File lib/logger.rb, line 590
    def shift_log_period(period_end)
      postfix = period_end.strftime("%Y%m%d") # YYYYMMDD
      age_file = "#{@filename}.#{postfix}"
      if FileTest.exist?(age_file)
        # try to avoid filename crash caused by Timestamp change.
        idx = 0
        # .99 can be overriden; avoid too much file search with 'loop do'
        while idx < 100
          idx += 1
          age_file = "#{@filename}.#{postfix}.#{idx}"
          break unless FileTest.exist?(age_file)
        end
      end
      @dev.close rescue nil
      File.rename("#{@filename}", age_file)
      @dev = create_logfile(@filename)
      return true
    end