Service matter

The position in the seniority list is based is based on the rank position in the selection process, not the date of joining. 

Even before joining, do not expose yourself as a litigant, first report, and using RTI, get such laid down rules and regulations that state about reckoning seniority either from DOPT and GAD of your state.  Decide it at a later as you can agitate it at any later date.  First focus on acquiring of skills and display commitment.  Do not expose yourself as litigant even before you join the institution

In this context the following instructions issued by Department of Personnel Training vide O.M.No.20011/1/2008-Estt.(D) dated 11.11.2010provides the guidance:

The relative seniority of all direct recruits is determined by the order of merit in which they are selected for such appointment on the recommendations of the U.P.S.C. or other selecting authority, persons appointed as a result of an earlier selection being senior to those appointed as a result of subsequent selection. The relative seniority that used to be determined earlier according to the date of confirmation and not the original order of merit, (in case where confirmation was in an order different from the order of merit indicated at the time of their appointment), in accordance with the general principles of seniority, has been discontinued w.e.f. 4.11.1992 (O.M. No. 20011/5/90-Estt (D) Dated 4.11.1992)

The instructions could be accessed at https://persmin.gov.in/DOPT/Seniority_Instructions_Guidelines/Seniority_Instructions_Guidelines.pdf

There is also an FAQ which could be accessed at https://dopt.gov.in/sites/default/files/FAQ_Seniority.pdf

If your selection for a post in the Central Government, your seniority would be governed by the above instructions. The State Governments are also following the same norms.

I agree with the opinion of Expert Mr. P. Venu