unknown
  %W   time of file birth, seconds since Epoch; 0 if unknown
  %x   time of last access, human-readable
  %X   time of last access, seconds since Epoch
  %y   time of last data modification, human-readable
  %Y   time of last data modification, seconds since Epoch
  %z   time of last status change, human-readable
  %Z   time of last status change, seconds since Epoch

