Registration under gst

If a person made taxable outward supply and his aggregate turnover during the year exceed 20 Lakh on a particular day lets say his turnover on 19-Jan-2018 was 19.75 Lakh and he made outward supply of Rs 1 Lakh on 20-Jan-2018 on this day he became liable to take registration under GST as his turnover exceed 20 Lakh. Case 1: He applied for GST registration on 05-Feb-2018 ie with in 30 Days and he received his GST registration on 09-Feb-2018. Case 2: What if he doesn't take registration within 30 days and applied GST registration on 25-Feb-2018 ie after 30 Days and he received his GST registration on 28-Feb-2018. From which date and on which amount he is liable to charge GST in both scenarios. Is he liable to issue revised invoice to his customer. Is he needs to file first return under GST? Please put the light on above query with relevant legal provision.