Seniority at Manager grade which date to be considered, 01 Jan 18 (when on contract) or 01 Jan 20?

Hi All, Need suggestion on the following.
A person joins my Company say as Manager grade on 01 Jan 2018 on 3 years fixed term contract. Before the same ends, Company decided to absorb that person in permanent roll at Manager grade w.e.f. 01 Jan 2020. Now regarding his seniority at Manager grade which date to be considered, 01 Jan 18 or 01 Jan 20.
If there is any Govt. guidelines or case reference then please mention.
Please suggest.

The date of absorption should be 01 Jan 20.
He was a contract employee earlier and terms of employment different from a permanent employee.

Dear Saptarshi,
Since yours is a private enterprise and the incumbent being a manager, I am of the view that you are the ultimate judge in the matter. If the person engaged at present on FTC basis doesn't have the managerial capabilities you expect of him, certainly you would not be inclined to take him on regular roll during the subsistence of the contract. Therefore, you can place him on regular cadre with retrospective effect from the date of his joining as an FTC manager by modifying the existing contract provided the service conditions are the same or similar.
Regarding his inter se seniority with existing managers, you can place it at the bottom of the seniority pertaining to 2018 or even effective from Jan1, 2020 as suggested by Mr.Rao. It all depends on your assessment of his potential to move up in the organizational hierarchy as well as the possible heart-burn it can create among the existing lot of your managers. If you are a person not interested in unnecessary managerial politics in your organization, you may prefer the suggestion of Mr.Rao.

Thanks for your replies. My Company is a Public Sector Undertaking. However when i am searching at internet i am getting Govt. guidelines on similar issues for direct Govt employees who have been absorbed after adhoc employment like deputation but unable to get any clear guidelines for our specific case like fixed term to a absorption in permanent roll and date of Seniority and thus interpretations are being made.
Kindly suggest accordingly because there is no point to implement something which is legally not viable and can be challenged.
Regards
DG

Dear DG,
Had there been an explicit mention about the type of your establishment in the post, it would have been easier for us to respond appropriately.
The service regulations of the Government employees from initial appointment to termination of employment are totally different from that of any public sector undertaking.
Recruitment to Central Govt posts is done through UPSC and Staff Selection Commission whereas for State Govt posts by the the State Service Commissions. In respect of Public Sector Undertakings, we have Railway Recruitment Boards, Public Sector Enterprises Selection Board ( Top Management cadre like Chaiman, M.D), Banking Services Recruitment Board and the like.
Main methods of recruitment are Direct Recruitment, Short term Contract, Promotion, Re-employment, Absorption, Deputation.
When you want to absorb an FTC employee in a post earmarked for the other modes like direct recruitment or promotion, first you have to ensure whether your service regulations permit such absorption and then fix the seniority in such a manner that the promotion avenues open to the existing seniors from the original stream are not affected.