No they are not wrong! Only the promo of "1990 Ki shaadi"
that was wrong. I suppose here they meant to say '90 s life shaadi - not a Shaadi that happened in 1990. Pooja's marriage happened in 1991 only. The marriage card was shown on instagram, it was never shown properly in the serial.
You all remember ?Rakesh was asking for 1991 calender fr om one if his students long back. And after hand holding segment, Sameer and Naina met after some days. He mentioned like "After meeting you in the school for whole day I am not satisfied, how to stay for two days? " Arjun was talking about college form. The farewell was for 1990 12th batch, not in 1990. Exam time has a confusion. Let's see which exam they appear for.