Am I the only person who got progressively more creeped out as it went along? I started out reading it thinking that the protagonist was a total wuss, and kind of annoying, and I honestly expected something simple, like Groundhog Day style stuff where a single day repeats over and over, and the protagonist eventually learns some moral lesson. But no.
It's actually an unsettlingly written manga that delves into occasional philosophical thoughts. The girl, Toko, is the kind of person who tends to want to run away from things, and on August 31, she really, really doesn't look forward to September 1st because of a reason that gets revealed slowly later on. Toko can also 'back-step'--travel backwards through time, but not intentionally. It happens when she wants to get away from something, or run away. It turns out her back-stepping isn't exactly time-travel backwards; it's more like sliding sideways into a parallel world, to a slightly earlier time relative to the current one.
The manga uses the premise that time is a fractal, where every single human being's possible action can be distilled to two paths (do or do not), and every single action thereafter branches the same way until there are infinite possibilities. Toko's ability to time-step can have hefty consequences on all the worlds, since apparently time's fractal construction can be interfered with by another fractal, if Toko herself is at the center of it. This is the reason why there is someone who is going after Toko, in an attempt to stop her.
The manga, in summary, is extremely complex. But the tangles do get somewhat untangled, and the knots are untied, if not severed at times, so reading through the manga mainly takes some patience (like another reviewer said), and possibly a taste for time travelling, nigh-paradox situations (Primer, anybody?). It manages to be unsettling as well, from the way it is written, and it generates a lot of this creepiness from the subtle and unsubtle changes in the characters as Toko backsteps through and into more and more parallel worlds where things turned out differently. All the 'what-ifs', and scenarios where you wished you did something, and you wondered how life might have turned out...it's that kind of feeling.
Read it if you have time. It isn't a cheap snack manga; it's a meal with very complex flavors, and if you lazily skim through it, you'll miss the meat.
Here's hoping for more translations of it!