Oops - well behind, once again. Evidence, if any were needed, that maths teacher is back at the electronic chalkface.
Anyway, The Time Traveler's Wife, based on the highly successful (and enjoyable) novel by the somewhat unfeasibly named Audrey Nifenegger. A tale of love and time travel, which takes the route of seeing time as being non-linear... a sort of big ball of wibbly-wobbly, timey-wimey stuff. Quite right too.
The main idea is of course that when our Time Traveler (minus an L for reasons known only to Americans) Eric Bana meets the always watchable eventual Wife Rachel McAdams for the first time (in his timeline), she's in her early twenties, yet has known him since she was about eight. Much to-ing and fro-ing ensues as their romance blossoms, and blossomed, and will blossom, to great joy and sadness. Why is she never visited by a particularly old version of EB? Uh oh.
The film is a decent effort at fiming a complicated book and manages the romance side of things pretty well - hankies at the ready, folks! But it doesn't really generate enough tension and perhaps gives away too much too soon.
Overall, a watchable effort. But I wouldn't travel back in time to watch it again.
How's the maths?
Well, it would be interesting to try and do a timeline for the two lovers, but as with any time travel or flashback movie, it's really all about the additive inverses within the integers. So if we head back, say, 15 years (negative fifteen) and then want to come back to the present day, then that's adding 15 to -15 to get 0.
But you spotted that already.