Collecting tax

from date of registration

From the date of approved your registration.

Liable from approval date of gst registration

you have to charge GST from where you are liable to charge gst, for e.g. if turnover exceeded Rs. 20lakh, you have charge gst on all invoice issued after 20lakh limit.
and if you have not received GST no. till that time, in that case you have to issues revised invoice to all buyers have purchased "from limit cross till GST no. received.