This girl reappears sometime later in the film, when Tim is out with one of his guy friends watching one of Harry's plays; Tim sees her in the audience and after the show's over, approaches her to say hello. He ends up walking her home and, when they reach her door, she propositions him. Tim looks at this woman whose physical appearance and personality had dazzled him- still do, honestly- who is now inviting him to sleep with her; he excuses himself, goes home and proposes to Mary. He doesn't need a do-over to realize his relationship with Mary is worth so much more than temporary gratification with someone who by her own choice doesn't know him, but is willing to sleep with him because he's a successful lawyer now. Being able to go back and change things afterwards is no replacement for choosing the right and moral path the first time around.
I said at the beginning of this post that I didn't think this movie would get made today, because the values it promotes are consistently the opposite of what modern Hollywood seeks to promote on screen. First of all, as mentioned, it does not portray men as inherently bad/stupid/useless. It in fact portrays fathers as positive role models. It also promotes strong families, getting married, and- gasp- having multiple children. That's right, in About Time, Mary has had two children with her husband Tim and wants more... oh no, it's like The Handmaid's Tale or something... she's been brainwashed by the patriarchy! From what I can see in most present day movies, a woman is only considered strong if she's- somehow- punching out a hulking man who outweighs her by about a hundred pounds before doing a backflip and dropkicking a couple more hapless dudes behind her. She should also be a hardened cynic, vaguely androgenous, and intellectually as well as physically unbeatable- while of course still being a victim of male privilege. Then you watch a film like About Time and think: egad, remember when not all women protagonists were seething with anger and resentment? I'm not saying that About Time is the most brilliant movie I've ever seen, by a long shot. But it is fun, it raises some interesting points to think about, and it's a refreshing change from the shallow yet dour, overwrought yet emotionally immature fare we're regularly served up these days.