def zulu(self):

        ''' a method to report ISO UTC datetime string from a labDT object

        NOTE: for timezone offset string use .isoformat() instead

        :return: string in ISO UTC format
        '''

    # construct ISO UTC string from labDT
        utc_dt = self.astimezone(pytz.utc)
        iso_dt = utc_dt.isoformat()
        return iso_dt.replace('+00:00', 'Z')