question regarding seniority

I was appointed as an adhoc LDC on 3/2/1992 and during this period vacancy was taken out and so exam held but my exam was exempted at that time temporarily and my others mates qualified. Then in August I also qualified my exam and now after such years they are saying that they are senior but according to the law who is senior, the one who appointed previously (as an Ad Hoc) or the one who qualified first .Till now all benefits and promotions had been given to me considering me as a Senior and please also give link regarding that ruling