Gst liability date

The taxpayer is a retail cloth merchant with sales above Rs. 20 lacs in 2016-17.

GST Reg. was applied on 28th July 2017 but Registration Certificate was issued dated 8th Aug. 2017. The date of liability row is not showing any date in the RC. The 3B form was showing from month Aug. 2017. Now from which date we need to file invoices in GSTR-1 (Jul-Sep Quarter). From which date the liability will arise - date of TRN or ARN or Date of RC? Please advise.