code: def convert_to_timestamp(time):
      
      if time == -1:
          return None
      time = int(time*1000)
      hour = time//1000//3600
      minute = (time//1000//60) % 60
      second = (time//1000) % 60
      return str(hour).zfill(2)+":"+str(minute).zfill(2)+":"+str(second).zfill(2)
docs: Convert int to timestamp string.