I thought the same too. But it is an informal interview invitation e-mail which has been sent before the deadline where you have to let them know which one of three dates is more convenient for you. There is not much to worry about if you still haven't received the same e-mail. Its not like they will not consider the late applicants. And the results of the interviews are declared only after the last interview is over.
hey ammy. thanx for that.. i was a bit worried.hope we get through. take care and all the best.