I am very sorry to differ from the views expressed by our learned friends M/S Vibhakar and Kamesh. The query in the original post about the impact of a belated declaration of probation on one's seniority is not about the isolated case of the appointment of a single employee and the declaration of his probation. On the contrary, it presupposes the situation of more than one employee getting appointed at the same time and placed on probation. When more than one person is selected and appointed to the required number of vacancies of the same cadre, their seniority is based on their merits, and the sequential order in the appointment letter conveys it. In government service, the difference in the dates of joining does not alter the original seniority assigned in the appointment orders.
For example, out of 5 candidates placed on probation on the same date, if the one with seniority position 2 fails to complete probation successfully and it gets extended, say, for a further period of 3 months, while others with seniority positions 1, 3, 4, and 5 get declared to have successfully completed probation. Just within a month, there arises a situation for filling up 3 vacancies in the immediate next higher post by promotion from the feeder category. Of the 4 approved probationers, only the employees with seniority numbers 1, 3, and 4 will be promoted.
Now, further imagine that the employee with seniority number 2 completes his probation successfully in the extended period, and immediately after a month or so, a further vacancy for promotion arises. When the question of merit issues like the ones mentioned by Mr. Rao is to be considered, the junior with the original seniority number 5 would be preferred over 2 by virtue of earlier completion of probation. In government service, one would be required to draw his second annual increment onwards only after the declaration of probation. In private employment, I suppose that no employer would grant an annual increment exactly on the original date due to an employee undergoing an extended period of probation. Thus, seniority becomes effective only from the date of confirmation for employment benefits such as increment, posting, and promotion.