code: def unixtime_to_datetimestr(unixtime, timefmt='%Y/%m/%d %H:%M:%S', isutc=True):
      
      try:
          if unixtime == -1:
              return 'NA'
          if unixtime is None:
              return None
          if isutc:
              return datetime.datetime.utcfromtimestamp(unixtime).strftime(timefmt)
          else:
              return datetime.datetime.fromtimestamp(unixtime).strftime(timefmt)
      except ValueError:
          raise
docs: TODO: ranme to datetimestr