Glitch

From the critically
acclaimed author of Float comes a new
whirlwind adventure about a pair of kids who must break all the rules of
time travel, perfect for fans of Gordon Korman and John David Anderson.

Regan Fitz and Elliot Mason have been enemies since they
started training to become Glitchers-people who travel through time to preserve
important historical events. But everything changes when they find a letter
from Regan's future self, warning them about an impending disaster that
threatens them ...