I actually really enjoyed this movie and am surprised to see so many negative, biased reviews for a product which is way above average compared to the hackneyed scripts we get to see these days.
The screenplay keeps you guessing till the very end and this is indeed a very smart adaptation of the "time-travel" concept. So much so that you will be compelled to view the movie again just to clarify your lingering doubts on how precise plot points were interconnected.
A note on the acting - the performances from most of the cast were top-notch, but Tapsee Pannu literally carried the film through on her shoulders. Ably supported by Pavail Gulati and the child actor who plays Aney.
Director Anurag Kashyap has ensured that this movie will be talked about a lot in times to come as a benchmark for future time travel movies coming out of the Hindi film industry!