The plot of About Time revolves (obviously) around time travel, and the film wisely makes no attempt to explain the phenomenon. Because there can be no rational explanation for it, and it's always going to be plagued with contradictions and paradoxes. So the movie has Bill Nighy's character tell his son Tim about their inherited power in his rather laconic, deadpan way and expects the audience to just accept the humorous non-explanation and go with it. Speaking of Bill Nighy, he's great as James, Tim's dad- stereotypically British, often hiding his feelings behind rather sardonic, self-deprecating humour, but a solid family man who Tim very obviously respects and loves very much.
It's also possible to question the morality of Tim keeping such a big piece of his life from Mary- especially when he's used his ability on occasion in ways which affect her life as well as his own. For example, when he went back in time to save his sister from her toxic relationship, Tim inadvertently changed their daughter to a son (see the movie summary) by his meddling with past events. If he hadn't gone back and re-changed things back to how they were originally, Mary would have lost her daughter and never have even known it. Which is rather a horrifying thought. I do give him a pass for intercepting Mary on her way to meet her new boyfriend, as Tim was in that case again returning events to how they were supposed to be, when he and Mary met first and became a couple.
About Time is a film in which the plot is powered by time travel, but the movie isn't really about that; it's about other, more important things. I had intended to fit my thoughts on the movie all into this post, but now I think I'll opine on them in a follow up post; I'm a bit pressed for time today and don't want to rush though it and forget half of what I want to discuss. I'll also discuss why I think that this film is superior to a lot of romantic comedies from the 2000's... not that that's a high bar.