This is from Glen Hiemstra of Futurist.com:

Mayan December is an exciting tale of the ancient Mayans and the end of the Mayan calendar in the near future, as seen through the eyes of a time-traveling youngster named Nixie.  What begins as an interesting trip to Chichen Itza for the December 2012 equinox becomes a pulse-pounding race against time as Nixie, her scientist mother Alice, other friends and eventually the Presidential Science Advisor and world leaders find themselves stepping back and forth between the past and the present.  Nixie’s uncomfortable dilemma is whether to intervene in the past.  Her challenge, she decides, is to save a young Mayan girl from ritual sacrifice, and in doing so perhaps save the world of the future.  Brenda Cooper’s novel is a fascinating exploration of the nature of time itself, a lesson in the history of the Mayan culture, and a speculation on what the famous equinox of December 2012 could mean, all wrapped up in a story of mystery, intrigue and danger.