About time is a beautiful romantic comedy with an important message about living life to the full, missed opportunities and making the best of every instant. As such it is a great example of using time travel as a literary means of exploring a human issue, just as robots are often used to explore what it means to be human, or parallel universes have been used to explore societal issues.
But About Time also plays around with timelines and most of the time this is done consistently. As is to be expected, this is not about the science, so the time travel mechanism is not explained in scientific terms - it just requires a dark room and clenching one's fists, and one gets to the time one is thinking of. However it is unfortunate that this consistency is broken unnecessarily in one of the key moments of the film. SPOILER ALERT.
First let us establish that the time travel mode of this movie: it seems that when one travels back one takes up one's own body at the moment in the past, re-living life for a short while. The main character, Tim, choses to go back in time to avoid his sister's car accident. When he comes back to the present he discovers that his daughter is no longer the one he had had in the previous version of events. From his father he learns then that when one goes back to before one's child is born, a different child is born, as it is very unlikely that it is the same sperm hat get to fertilise the egg at conception. So he decides to relive the moment when he travelled back in time and not do it - with the intention of therefore allowing his sister's accident to happen in exchange of restoring his own child to the original one. This is where the inconsistency takes place: NOT travelling in time in THIS timeline will not restore the previous one, it will just keep things as they are, as we are not in a scenario of a unique single timeline where all time travel is already built-in (=already "happened"), but in a scenario where each time travel event generates a new timeline. But as said, this does not detract from the clever use of time-travel to formulate a great message about love, life and, well, time.