GSTP.

Those covered under Rule 83 (1)(b) i.e. those covered as under existing law as Sales Tax Practitioner or Tax Return Preparer for not less than 5 years only i.e. those who have completed 5 years or more are required to answer on or before 31.12.2018.
So they need to enroll and answer on 31.10.2018.
For others they can answer on later date. For them time limit of 31.12.2018 is not applicable.

Correct me if I am wrong.

Yes you are correct dear Piyush

The GSTPs enrolled on the GST Network under sub-rule (2) of Rule 83 and covered by clause (b) of sub-rule (1) of Rule 83, i.e. those meeting the eligibility criteria of having enrolled as sales tax practitioners or tax return preparer under the existing law for a period not less than five years, are required to pass the said examination before 31.12.2018 in terms of second proviso to rule 83(3). The examination for such GSTPs shall be conducted on 31.10.2018 from 1100hrs to 1330 hrs at designated examination centres across India.

Hence Exam conducted on 1.10.18 is only for STP (Sale tax practitioner) & TRP who is practising for not less then 5 yrs . 

Agreed with Shri Pankaj and Piyush

@ Pankaj Are you answering GSTP? I mean do you fall under the category of 83(1)(b)?