As per Section 40, a registered taxable person is required to file First Return to disclose the details of supplies effected during the period between the date on which he became liable to registration till the date on which registration is granted. Therefore, the assesse will disclose the details from 12.07.2017 to 16.08.2017 in the August Return.
(ICAI FAQ PUBLICATIONS 06-09-2017 RETURNS : FAQ NO. 27)