The GSTR1 is supposed to contain details of all outward supplies made by a registered person.
The government has issued a notification notifying the last dates for filing of GSTR1 for the months of July and August. The last dates are:
For July, 2017: 1st to 5th September, 2017
For August, 2017: 16th to 20th September, 2017.
Find the notification below: