def posix_to_dt_str(posix):
  dt = datetime.datetime.utcfromtimestamp(posix)
  dt_str = dt.strftime(_DT_FORMAT)
  return dt_str + '.000Z'
Reverse of str_to_datetime.
dataset/ETHPy150Open AppScale/appscale/AppServer/google/appengine/ext/cloudstorage/common.py/posix_to_dt_str