Length of service

So, you are keen to know with regard to seniority and promotion!!!

First of all, the seniority has to be reckoned with reference to the date of appointment in a particular cadre. Here, you were appointed in the cadre of "Clerk", whereas, the post that was advertised and filled up is a post higher than the clerk. The employer has a right to fill in the post either by promotion, or by direct recruitment. But, promotion from lower post to higher post depends upon the rules relating to conditions of services and or promotion. Admittedly, as said by you, there is no channel of promotion in the society. Hence, you cannot claim promotion. (However, the matter of promotion depends upon the rules framed)

You could have applied in response to the advertisement and preferred to appear before the authority, provided you were possessing the required qualifications and experience. It appears, you have missed the Bus. There is hardly any substance in the case, and accordingly, I do not think that you have a sound case on merits. Even if you agitate, the claim in all probabilities will be dismissed as "totally meritless". 

your qury is vague.  You have not clarified whether you have actually applied and selectged to higher post.

anyway let it bne clear that length of service and seniroity are different aspects. One may be having 20 years length of service but only 6 mnths seniority in present post.

One may be having more legth of service but being lower merit will be having a lower sernioiryt than the later joinee having better position in merit.

You come with facts what you want to knpw

Basically my question is length of Service and seniority on the post is different or not 

And I need citation/authority on the point.

it is clear that length of service and sniority may or may not be same.

you are likely to keep getting incomplete (even wrong advise) unless you disclose full facts.