The Here and Now, by Ann Brashares

Prenna and her mother live in a tight secretive community that lives by twelve key rules designed to keep them at arm's length from others.  It's necessary to protect the "time natives" of the present from contamination from their community of time travelers.  After all, they all understand what the future will bring:  global warming and plagues.  No matter how tough life might seem now, it is paradise compared to where these people come from!

But Prenna discovers that there is even more danger in not interfering. With the help of a time native (i.e., a particularly talented classmate named Ethan), she is trying to influence a series of events that form a "fork" in the continuum.  This juncture will take place very soon and decide whether the future will turn into nightmare or whether it will take an unknown alternative path.

It's a strange choice of genre for an author most strongly associated with those amazing traveling pants!  And, while she struggles a bit with the usual rigor expected in science fiction, Brashares does a surprisingly good job with this time travel yarn.  That's mostly because she keeps the story very much in the present and (mostly) in New Jersey.  In her telling of the tale, Prenna and her friend Ethan are just a bunch of normal, impulsive teens.  The focus is strongly on their relationship, while the high stakes adventure takes a back seat.  Even though the pacing is brisk, its doesn't stop Prenna and Ethan from having time to hang out and even walk on the beach as the world hurtles towards its critical moment.